OK. You turned 50. You know you're supposed to get a colonoscopy. But you haven't. Here are your reasons:

1. You've been busy.

2. You don't have a history of cancer in your family.

3. You haven't noticed any problems.

4. You don't want a doctor to stick a tube 17,000 feet up your butt.

Let's examine these reasons one at a time. No, wait, let's not. Because you and I both know that the only real reason is No. 4. This is natural. The idea of having another human, even a medical human, becoming deeply involved in what is technically known as your ''behindular zone'' gives you the creeping willies.

I know this because I am like you, except worse. I yield to nobody in the field of being a pathetic weenie medical coward. I become faint and nauseous during even very minor medical procedures, such as making an appointment by phone. It's much worse when I come into physical contact with the medical profession. More than one doctor's office has a dent in the floor caused by my forehead striking it seconds after I got a shot.

In 1997, when I turned 50, everybody told me I should get a colonoscopy. I agreed that I definitely should, but not right away. By following this policy, I reached age 55 without having had a colonoscopy. Then I did something so pathetic and embarrassing that I am frankly ashamed to tell you about it.

What happened was, a giant 40-foot replica of a human colon came to Miami Beach. Really. It's an educational exhibit called the Colossal Colon, and it was on a nationwide tour to promote awareness of colo-rectal cancer. The idea is, you crawl through the Colossal Colon, and you encounter various educational items in there, such as polyps, cancer and hemorrhoids the size of regulation volleyballs, and you go, ''Whoa, I better find out if I contain any of these things,'' and you get a colonoscopy.

If you are as a professional humor writer, and there is a giant colon within a 200-mile radius, you are legally obligated to go see it. So I went to Miami Beach and crawled through the Colossal Colon. I wrote a column about it, making tasteless colon jokes. But I also urged everyone to get a colonoscopy. I even, when I emerged from the Colossal Colon, signed a pledge stating that I would get one.

But I didn't get one. I was a fraud, a hypocrite, a liar. I was practically a member of Congress.

Five more years passed. I turned 60, and I still hadn't gotten a colonoscopy. Then, a couple of weeks ago, I got an e-mail from my brother Sam, who is 10 years younger than I am, but more mature. The email was addressed to me and my middle brother, Phil. It said:

``Dear Brothers,

``I went in for a routine colonoscopy and got the dreaded diagnosis: cancer. We're told it's early and that there is a good prognosis that they can get it all out, so, fingers crossed, knock on wood, and all that. And of course they told me to tell my siblings to get screened. I imagine you both have.''

Um. Well.

First I called Sam. He was hopeful, but scared. We talked for a while, and when we hung up, I called my friend Andy Sable, a gastroenterologist, to make an appointment for a colonoscopy. A few days later, in his office, Andy showed me a color diagram of the colon, a lengthy organ that appears to go all over the place, at one point passing briefly through Minneapolis. Then Andy explained the colonoscopy procedure to me in a thorough, reassuring and patient manner. I nodded thoughtfully, but I didn't really hear anything he said, because my brain was shrieking, quote, ``HE'S GOING TO STICK A TUBE 17,000 FEET UP YOUR BUTT!''

I left Andy's office with some written instructions, and a prescription for a product called ''MoviPrep,'' which comes in a box large enough to hold a microwave oven. I will discuss MoviPrep in detail later; for now suffice it to say that we must never allow it to fall into the hands of America's enemies.

I spent the next several days productively sitting around being nervous. Then, on the day before my colonoscopy, I began my preparation. In accordance with my instructions, I didn't eat any solid food that day; all I had was chicken broth, which is basically water, only with less flavor. Then, in the evening, I took the MoviPrep. You mix two packets of powder together in a one-liter plastic jug, then you fill it with lukewarm water. (For those unfamiliar with the metric system, a liter is about 32 gallons.) Then you have to drink the whole jug. This takes about an hour, because MoviPrep tastes -- and here I am being kind -- like a mixture of goat spit and urinal cleanser, with just a hint of lemon.

The instructions for MoviPrep, clearly written by somebody with a great sense of humor, state that after you drink it, ''a loose watery bowel movement may result.'' This is kind of like saying that after you jump off your roof, you may experience contact with the ground.

MoviPrep is a nuclear laxative. I don't want to be too graphic, here, but: Have you ever seen a space shuttle launch? This is pretty much the MoviPrep experience, with you as the shuttle. There are times when you wish the commode had a seat belt. You spend several hours pretty much confined to the bathroom, spurting violently. You eliminate everything. And then, when you figure you must be totally empty, you have to drink another liter of MoviPrep, at which point, as far as I can tell, your bowels travel into the future and start eliminating food that you have not even eaten yet.

After an action-packed evening, I finally got to sleep. The next morning my wife drove me to the clinic. I was very nervous. Not only was I worried about the procedure, but I had been experiencing occasional return bouts of MoviPrep spurtage. I was thinking, ''What if I spurt on Andy?'' How do you apologize to a friend for something like that? Flowers would not be enough.

At the clinic I had to sign many forms acknowledging that I understood and totally agreed with whatever the hell the forms said. Then they led me to a room full of other colonoscopy people, where I went inside a little curtained space and took off my clothes and put on one of those hospital garments designed by sadist perverts, the kind that, when you put it on, makes you feel even more naked than when you are actually naked.

Then a nurse named Eddie put a little needle in a vein in my left hand. Ordinarily I would have fainted, but Eddie was very good, and I was already lying down. Eddie also told me that some people put vodka in their MoviPrep. At first I was ticked off that I hadn't thought of this, but then I pondered what would happen if you got yourself too tipsy to make it to the bathroom, so you were staggering around in full Fire Hose Mode. You would have no choice but to burn your house.

When everything was ready, Eddie wheeled me into the procedure room, where Andy was waiting with a nurse and an anesthesiologist. I did not see the 17,000-foot tube, but I knew Andy had it hidden around there somewhere. I was seriously nervous at this point. Andy had me roll over on my left side, and the anesthesiologist began hooking something up to the needle in my hand. There was music playing in the room, and I realized that the song was Dancing Queen by Abba. I remarked to Andy that, of all the songs that could be playing during this particular procedure, Dancing Queen has to be the least appropriate.

''You want me to turn it up?'' said Andy, from somewhere behind me.

''Ha ha,'' I said.

And then it was time, the moment I had been dreading for more than a decade. If you are squeamish, prepare yourself, because I am going to tell you, in explicit detail, exactly what it was like.

I have no idea. Really. I slept through it. One moment, Abba was shrieking ``Dancing Queen! Feel the beat from the tambourine . . .''

. . . and the next moment, I was back in the other room, waking up in a very mellow mood. Andy was looking down at me and asking me how I felt. I felt excellent. I felt even more excellent when Andy told me that it was all over, and that my colon had passed with flying colors. I have never been prouder of an internal organ.

But my point is this: In addition to being a pathetic medical weenie, I was a complete moron. For more than a decade I avoided getting a procedure that was, essentially, nothing. There was no pain and, except for the MoviPrep, no discomfort. I was risking my life for nothing.

If my brother Sam had been as stupid as I was -- if, when he turned 50, he had ignored all the medical advice and avoided getting screened -- he still would have had cancer. He just wouldn't have known. And by the time he did know -- by the time he felt symptoms -- his situation would have been much, much more serious. But because he was a grown-up, the doctors caught the cancer early, and they operated and took it out. Sam is now recovering and eating what he describes as ''really, really boring food.'' His prognosis is good, and everybody is optimistic, fingers crossed, knock on wood, and all that.

Which brings us to you, Mr. or Mrs. or Miss or Ms. Over-50-And-Hasn't-Had-a-Colonoscopy. Here's the deal: You either have colo-rectal cancer, or you don't. If you do, a colonoscopy will enable doctors to find it and do something about it. And if you don't have cancer, believe me, it's very reassuring to know you don't. There is no sane reason for you not to have it done.

I am so eager for you to do this that I am going to induce you with an Exclusive Limited Time Offer. If you, after reading this, get a colonoscopy, let me know by sending a self-addressed stamped envelope to Dave Barry Colonoscopy Inducement, The Miami Herald, 1 Herald Plaza, Miami, FL 33132. I will send you back a certificate, signed by me and suitable for framing if you don't mind framing a cheesy certificate, stating that you are a grown-up who got a colonoscopy. Accompanying this certificate will be a square of limited-edition custom-printed toilet paper with an image of Miss Paris Hilton on it. You may frame this also, or use it in whatever other way you deem fit.

But even if you don't want this inducement, please get a colonoscopy. If I can do it, you can do it. Don't put it off. Just do it.

Be sure to stress that you want the non-Abba version.