Think you've been to your last Be-In? Smiled at your last flower child? Let the sunshine in until you were blue in the face? Not quite. "Hair" is back.

In case you've managed to forget, that would be "Hair," the counterculture musical that sent the theatrical world into a tizzy when it had its Broadway opening 10 years ago this month. "It is vulgar, perverted, tasteless, cheap, cynical, offensive and generally lousy," wrote the drama critic of the August New York Daily News, "and everyone connected with it should be washed it strong soap and hung up to dry in the sun."

Which shows how much he knew, because "Hair" went on to make packets and packets of money and is just now being finally turned into a $10-C-movie. And right here in Washington, D.C. to boot.

At the Lincoln Memorial end of the Reflecting Pool, a 60-foot-by-40-foot-by-4-foot-high platform has been built on which starting tomorrow morning at 10 a.m., three musical numbers will be filmed, featuring the Twyla Tharp dancers - "3-5-0-0," "What a Piece of Work is Man" and (of course) "Let the Sunshine In." And everyone, particularly younger types, is invited to participate.

"It is suggested that those who attend dress tha part," reads the press release, "that is, in the costume of a flower child or dressed casually in blue jeans, work clothes, schoolwear, etc." Consider yourself warned.

"My original idea was 200,000 people," said director Milos Forman at a rehearsal yesterday, gesticulating grandly with his cigar as he watched the dancers undulate across the stage, "but I was toned down to 20,000. Still, I hope we get 30,000.

A burly, bemused man. Forman looks more like a contender for the middleweight championship of Czechoslovakia than a director whose last film, "One Flew Over the Cuckoo's Nest," was ferociously successful both at the box office and at the Academy Awards. Why, when anything he desired could be his, had he chosen "Hair?"

I've been waiting for a chance to do it for years," he replied with enthusiasm. "I saw 'Hair' at its first previews off-Broadway in 1967 and immediately I went backstage, talked to the authors, got the music and brought it back to Czechslovakia, where I tried to get someone to put it on even before it went on in America."

That was a long time ago, however, and Forman himself is "surprised that I'm still excited. Of course it was not 10 years, every day. Every two or three years the feeling came back, I sat down and played the record and it was still exciting for me."

As to what exactly it is that continues to excite him, Forman only shrugged. "It's like a woman," he said finally. "Why am I in love with this one and not that one? It just has that indescribable sex appeal."

And like anyone in love, the director finds it hard to believe that the object of his affection just might be dated, that nobody wants to see another soulful screen hippy if he can possibly help it.

"If you worried about things like that, you'd never do anything, you'd never get out of bed in the morning," Forman said reasonably enough. "It is exciting for me, so I decided to take the risk that I'm not so different from everyone else."

Also keeping the faith is producer Lester persky, who said, "I hope it won't be any more dated than when 'All Quiet on the Western Front' came out 13 years after World War I. It would be more dated if we made it in 1968. When everyone thought that world would never end."

This is apparently the basic tenet of the "Hair" people, that its subject matter has aged superby like some wonderful claret. "Five or 10 years ago, it might have been more trendy, more merchandisable, but relying on the faddism of the hippy movement could be a trap," Persky said. "Now we've had time to seek more meaningful things, we've had a chance for reflection."

As to what all this reflection has done to "Hair's" minimal stage plot, no one is saying.

"I don't want to talk much about these things, but what works on the stage does not always work on film," said Forman, with Persky adding, "If we could tell the story in a few sentences, we wouldn't take the trouble to make the movie. It isn't just the plot line, it's the total experience of what the movie says. I can't say anything other than we're making the film, we have faith."

Peace and love, brother, peace and love.