ON A CLEAR spring night in 1969, Ben Barenholtz nervously paced the lobby of his Elgin Theater in New York's Chelsea district. Midnight was approaching, and the new audience was slowly drifting in for a surreal Mexican western that had never been screened outside a museum. Opening the film at the midnight hour and consigning it to the wee hours was a bold strategy, but Barenholtz couldn't justify risks beyond the $24-a-week ad in the Village Voice that simply read "El Topo at Midnight-at the Elgin." The film's success -- it ended up playing that slot nightly for two years -- opened up a new philosophy of showcashing films.

Washington, January 1970: one film could be seen at midnight at this time of the year; by mid-1975, the number had crawled to four. Last week, 40 screens were showing "Midnight Movies" different from their normal nightly fare, with another two dozen adding midnight screenings of their regular offerings.

"Midnight Movies" -- people talk about them, and obviously many people go to see them, mostly on weekends in suburban shopping malls. Sometimes, they outgross first-run movie screenings. The New York-based Alternative Film Society coordinates 400 midnight screenings every weekend across the country; another 600 theaters organize their own shows, mostly rock and horror films as well as popular items that have dropped off the first-run circuit.

But the heart and soul of the Midnight Movie audience is the hardcore and groundbreaking fans of the genre's cult classics -- "Rocky Horror Show," "Night of the Living Dead," "Pink Flamingos," "The Harder They Come," "Eraserhead" -- who troop in time after time to see generally low-budget films whose scripts they have memorized line for line. Some fans have seen "Rocky Horror" 300 times, while the film itself has grossed close to $20 million, almost all in midnight situations. Outside of "Star Wars," a phenomenon in its own right, these films generate the only habitual, repeat audience in theaters today, as well as providing an alternative outlet for oddball independent films.

"But a distinction must be made," says Barenholtz, the man who fatered the movement. "What we did was take a feature film that hadn't had any exposure (outside a museum) and essentially create an alternative way of opening a film and getting it to an audience," without backing or vast ad expenditures.

As the owner of a small bootstrap operation in an out-of-the-way film house, Barenholtz was faced with theater economics: "It's there 24 hours a day, and it's really only being used 12 hours." He had seen Alexandro Jodorowski's film at the Museum of Modern Art: "half the people walked out, but I was fascinated by it, I thought it was a film of the time, that there was an audience for it -- but not the general film audience."

"We made it a discovery thing," Barenholtz adds, pointing out one of the distinguishing marks of the real "Midnight Movie" as opposed to a movie that simply plays at midnight. "The true midnight cult film is a phenomenon, there have really only been five or six. There are other kinds of films that can play at midnight and draw an audience, but not on a sustained basis."

The audience for "El Topo" took that rather surreal western to heart and mind. "They were very sensitive about it, talked about it. And it didn't open for reviews for months," although it soon sold out seven midnights a week strictly on word-of-mouth. "Within two months, the limos lined up every night because it became a must-see item."

Although the Janus had a late-night Film Society in the early '60s that screened experimental and/or obscure foreign films, the Biograph was the first Washington theater to host midnight movies with "Reefer Madness" in late 1969. Other cities soon followed suit. In New York, one of "El Topo's" biggest fans was the late John Lennon, who had his financial adviser, Allen Klein, buy it for wider distribution.A normal break, including a huge Times Square billboard, was a total failure, particularly compared to the results of Barenholtz' $24-a-week outlay. "A big ad budget defeats the purpose," he says. "The audience delights in discovery."

The next cult classic was John Waters' notorious and salacious "Pink Flamingos," the first midnight movie to get national exposure. Waters, the Baltimore filmmaker who says he "makes exploitation films for art theaters," won't say how much late-night money he's made off the film that cost him only $12,000, though he points out that an advertising campaign for a normal commercial engagement in New York would have cost more than the film itself.

During the early '70s, Waters was a mobile midnight movie mogul, driving around the country with prints in his car trunk, stopping in at various cities, "looking up the theaters with the weirdest movies and arranging my own screenings." He was also a part of Mike Getz' Underground Cinema 12, a network of 17 theaters across the country that organized midnight screenings of experimental films, mostly on a one-shot basis.

Waters, maker of less successful movies like "Mondo Trasho," "Multiple Maniacs" and "Female Trouble," feels his movies "don't need to make so much money right away," a philosophy which would not make him welcome in Hollywood. m"Flamingos" has been playing in a Los Angeles theater for eight years, the same period "The Harder they Come" has been in a Cambridge theater. Runs of three to five years are not uncommon for the classics, another factor that distinguishes them from the run-of-the-weekend midnight screenings.

The champion for expansive longevity is certainly "Rocky Horror," a high-camp theatrical hit filmed with a decent budget in 1974, released to an indifferent box office in 1975 and resuscitated at New York's Waverly in the spring of 1976. Today the film's 200 prints are in use every weekend midnight around the country (it's been at the Key in Georgetown three years as of this weekend), and a whole movement has grown up around the film, complete with its own fanzines. As Jacqueline Susann said, "once is not enough," and the Rocky fanatics bear her out. Some of the regulars, who imitate the dress and manners of the film's garish characters and participate in the film's dialogue, eventually become a part of the film and the theater's fabric.

Jonathan Rosenbaum, a film critic who is doing a book on the midnight movie phenomenon, wrote in his recently published autobiography, "Moving Places: A Life at the Movies" that "the film ["Rocky Horror"] is a work of art whose audience and creators are essentially one, grouped around a shared irony about sexual roles and social taboos that remains as a legacy of the sixties, and whose social function is largely to bring together strangers and open up new channels of communication between them. It resurrects movie-going as a communal event, making one proud, not embarrassed, to be sitting next to other people in the dark."

Barenholtz supports the parameters of the cult classic as "discovery, the in element, the shared experience and audience participation," a formula so vague that it's little wonder the major film companies have not tried to exploit the midnight movie market beyond leasing already existing films. With the exception of "Rocky Horror," the grosses are still small potatoes, though Barenholtz, who now runs independent-oriented Libra Film Distributors, points out that "a $500,000 gross on a film can make money for us and the film's producers; the majors only think in millions. And besides, the midnight cult film is unique. You can't just say 'I'm gonna make one."

He also points out that none of the directors of cult classics have ever repeated their successes, though George Romero came close to "Night of the Living Dead" with "Martin," and John Waters has gone on to increasingly bigger budgets ($65,000 for "Desperate Living" in 1977 and $300,000 for "Polyester," due this year and starring Tab Hunter and the transexual giant, Divine.)

That inexplicable key to success hasn't stopped California avant-rockers Oingo Boingo from trying to make a film, "Forbidden Zone," directed at the midnight audience. "And it looks it," snorts Barenholtz. "It's not going to make it.It's been tried by younger filmmakers before." To which John Waters replies proudly, "all my films are midnight movies."

Those close to cult movies often try to distance their films from ubiquitous midnight fare. They see the glut of rock and horror films as little more than a marketing ploy made possible by their devotion. Alternative Film's Roger Grod agrees, saying "the word cult is a very major problem." His organization concentrates on such perennials as "Clockwork Orange," "Woodstock," "Gilda Live," "The Song Remains the Same," "Fantastic Animation Festival." "Our audiences either want to rock and roll, be frightened a bit or see a drug-type movie," he says pragmatically. "And it's good for the exhibitors. It's an extra revenue, particularly when they have to put up such tremendous guarantees for first-run films. They need to squeeze every penny."

Ironically, squeezing pennies ended the life of the first midnight classic, "El Topo." After Allen Klein took the film over and failed with it, it was withdrawn from the market, and now, says one theater owner, it's overpriced. But Barenholtz adds that "it was strictly a product of the '60s. It wouldn't make a dime today." Especially if it had to compete with the midnight madness that it unleashed almost singlehandedly.