It's time for a small exercise in participatory journalism. Put down the paper for just a moment and run into your bathroom and grab a bottle of shampoo -- the one that's on the edge of the tub, still dripping wet from your morning shower.

Let's say you use Prell, the green shampoo in the funny-looking tube, which is Procter & Gamble's best-seller. Printed clearly on the tube are these instructions: "Wet hair well. Lather. Add a little more water to fully develop lather. Rinse. Repeat."

What if you have a small problem that you don't like to admit to even your best friends and are forced to use Head & Shoulders, another Procter & Gamble product, which is also America's favorite dandruff shampoo? There are those pesky instructions again: "Lather-Rinse-Repeat."

Begin to see the pattern? Why do most shampoos insist that you "Repeat"? Is there anything more here than a diabolical Madison Avenue plot to get us to wash our hair twice and thereby sell double the amount of shampoo?

It's easy to picture the humble beginnings of this global conspiracy. It's late 1958 and the mood is grim in the executive suites at Procter & Gamble. Top-secret forecasts have projected that -- because of the rise of the crew-cut -- shampoo sales will flatten out for years to come.

Arrayed around a long boat-shaped conference table are the marketing geniuses who run the Procter & Gamble empire. Each is wearing a sincere tie and a worried frown as he calculates what a bear market in shampoo will do to the chances of his children attending the college of their choice.

Suddenly, way down at the end of the table, a little guy named Quimby, whose job no one can remember, timidly raises his hand. "Excuse me," he begins in a quavering voice, "I have an idea how we can sell twice as much shampoo. Just instruct people to wash -- and then repeat."

Pandemonium at Procter & Gamble. When the dust settles, shampoo history has been made and Quimby has finally achieved his most cherished ambition -- an office with a window.

Admittedly, as conspiracies go, this doesn't rank with the second gunman theory or the Trilateral Commission. But a conspiracy is a conspiracy and, in this post-Watergate era, we've got to make do with what we've got.

I called Procter & Gamble and demanded an explanation, invoking the First Amendment, John Peter Zenger and Woodward and Bernstein.

Public relations department spokesman Kevin Donnellon responded very carefully: "These instructions have been on our labels as long as anyone here can remember." Ah-ha, they've buried the secret with Quimby. "You see," Donnellon continued, "the first application removes the dirt, while the second shampooing gives you even more lather and cleanliness." But, Donnellon also admitted, "We've discovered that as people today are shampooing more frequently; they're learning that they don't have to follow these instructions."

I next tried Helene Curtis, makers of Suave, the low-priced shampoo that has zoomed to first place in this competitive field. I asked Charles Cooper, president of their consumer products division, why the label on Suave Golden Shampoo says, "Apply, lather, rinse, repeat."

Cooper, befitting his elevated position, seemed to appreciate the magnitude of my discovery. "In talking to our lab," he said, "I've discovered it's traditional. They've been saying it from the Year One. I guess it's there for dirtier hair. It's sort of there as insurance. I shampoo every day, and I do it just once. But women with long hair should probably do it twice, as should teen-agers with oily hair."

It was just about then that I discovered the awesome power of the press. "Should the labels be revised?" Cooper asked, almost to himself. "I suppose it's a good idea."

On fire with reformist zeal, I quickly called three other shampoo manufacturers -- each with that pivotal word "repeat" on their labels. Each conversation followed the standard pattern: a reference to the glorious history of shampoo labeling, the contention that the lather is better the second time around and a lack of hard evidence that your hair actually gets cleaner if you wash it twice.

At Johnson Products, which sells Ultra Sheen shampoo primarily to black consumers, Grayson Mitchell said, "In the first lather you remove surface grease and dirt, and in the second lather, you get rid of residual dirt. A second shampooing also increases the absorption of the conditioner into the hair shaft." As for the label instructions, Mitchell added, "We treat them very importantly as guidelines for use. "We're a little different in that regard, because we're a black company."

When I originally asked Stuart Saunders, the public relations director at Gillette, if the word "repeat" was on the bottle of White Rain just to sell twice as much shampoo, he said. "I don't mean to be obtuse, but that's what I think is the answer."

However, Saunders called back five minutes later to announce, "I've just talked to someone in our personal care division. There is a greater cleansing action on the second shampoo." Five minutes later and there was a third call from Saunders: "I've talked to one of our shampoo experts, and I got a more succinct explanation for you. You get better cleaning action that way. I'm convinced, and I hope you're convinced."

I was almost convinced, until I noticed that Clairol had three different types of instructions on three different shampoos that they manufacture. Herbal Essence had no instructions whatsoever on their label, just florid words extolling their natural ingredients. Short & Sassy tells its users to "repeat, if needed." And their newest shampoo and best-seller, Clairol Condition Shampoo, explicitly tells you to "repeat" without any qualification whatsoever.

Phyllis Klein, Clairol's publicity director, gamefully tried to set me straight. "We originally had the line 'repeat' on Herbal Essence, but a few years ago, when we reformulated the product, we needed the space on the label for other purposes. As for Short & Sassy, it's an exceptionally rich and thick shampoo. We introduced Clairol Condition Shampoo about a year ago. It only comes in one size, 16 ounces, a really big bottle, and we had room on the label, so why not?"

Racked with anguish that major corporations were this cavalier about the labels on their shampoos, I called the office of Michael Pertschuk, the chairman of the Federal Trade Commission. Despite my fantasies of a Pulitzer, as a loyal American I was willing to share the fruits of my investigative reporting with the federal government. I asked the FTC what they thought and, more important, what they planned to do about this outrage perpetrated on the American consumer.

This was the FTC's official response to my shampoo investigation: "Off the top of our heads, we have no comment."