WHEN Robert Downey was working as the "house freak" in a Madison Avenue ad agency a few years ago, an expert was paid $28,000 to come and tell the agency how to sell a particular beer. When the big day arrived, this motivational research guy came before the board and said just three words: "Beer is cock." He then collected his stash and went home-and the agency, after pondering these words of wisdom, came up with a campaign about the beer "with the five-minute head."

This is the kind of thing that makes America great, and this is the kind of material that Robert Downey has incorporated into his new film, Putney Swope .

In case the word hasn't reached you yet, this film (now at the Paris Cinema) is the story of what happens when a bunch of black militants take over an ad agency. But the film isn't really about advertising or blacks. Putney Swope is rather a monument to chaos, a plea for insanity, an adversisement for anarchy.

"A movie can't change anybody; it can only confirm what you already believe," Downey said in an interview last week. "I want to convince people who are insane that they're right, and should stay with it."

And of course, Robert Downey is insane. He has had the distinction of being kicked out of four schools, his house, the U.S. Army, and a waiter's job at Howard Johnson's (He yelled back orders like "veal parmegian, hold the veal."). He is the all-American guy gone wrong. He is nuts. He is also right.

As Downey sees it, America is a land of systems like Madison Avenue-systems we should avoid like the plague. In his movie, a bunch of revolutionary blacks find themselves unwittingly sucked into a system they would like to destroy.

How do we escape? "Everything established should go out the window," Downey says, "But I don't know how to do it. If someone knew how, it would have happened by now."

So it hasn't happened and we must survive. Putney Swope -a film literally bursting at the seams with irrelevancies, obscenities, improvisations and illogical editing-tells us how. To avoid systems, we must act as if they don't exist; we must see how funny all that supposed logic is. We should wear lampshades on our heads, urinate in the kitchen, say "Fuck you" to any system that tries to encroach on our humanity. James Thurber would understand this kind of approach to life. And so would acid heads, people with good peripheral vision, and Lenny Bruce.

PUTNEY SWOPE is a fantastic movie, but people who still believe in the alphabet and political movements are going to have a lot of trouble with it. While the film does have some moments of pure comic entertainment value (like a lot of obscene commercials made by the militants' "Truth and Soul" advertising agency), it is not enough of a comedy to hold an audience for laughs alone.

Swope is a movie that literally disintegrates as you watch it-like Alka Seltzer. It defies all systems by not explaining its plot, its jokes or its characters. Most of the people in it are not only new to movies but new to acting.

Robert Downey, though, is not new to films. He is known to underground film audiences for such earlier, shorter works as Babo 73 and Chafed Elbows . He fought for the chance to do Swope for two years, carrying the script around to backers.

Once he got the money (from a fertilizer magnate), he worked an exhausting schedule to get the movie done. All of the sequences set in ad agency offices had to be filmed at night in closed office buildings. (The board room of the agency in the film is actually the board room of David Rockefeller's bank.)

Downey refers to himself as "a prince." "I'm too young to be a king and too committed to be a queen" is the stock answer he gives for this. But, he says a few seconds later, "you have to be a prince to survive all the shit in this business-running around for two years, kissing people's asses to get backing."

HE IS NOT entirely happy with the direction the new found enthusiasm for films in the United States is going. He feels there is something wrong with critics and filmmakers who dwell on technique to the extent of eliminating content in movies.

"Godard is a phony to me," he says. "His technique is great, but he just keeps replaying the same political rhetoric."