Exactly 20 years ago yesterday S. Dillon Ripley, then a few months into his epoch-making tenure as secretary of the Smithsonian Institution, established something called Smithsonian Resident Associates to link the public at large more directly with America's most diverse museum.

Ripley's prescription, predictably for the "Nation's Attic," seemed conveniently open-ended. The associates, said Ripley at the time, should "serve as a link between what the Institution does . . . and what the public in the Washington area can do to participate." What was it for? Public relations? Education? Entertainment? All of the above?

However irregularly the program may have evolved, the lectures, tours and classes of the Smithsonian Associates have become one of the most vital cultural forces on the local scene. They now touch an estimated 270,000 lives each year -- having, in their best moments, an almost familial character. Those involved range in age from the little ones who jam the Air and Space Museum at its annual Halloween party to hundreds of adults enrolled in the three-month "Tuesday Mornings at the Smithsonian" series that includes complimentary breakfast. "We're growing from womb to tomb," quipped program director Janet W. Solinger in a joint interview with some of her staff early this week.

And last night, hundreds gathered for an anniversary party at the red brick, tile and turreted rotunda of the glorious old Arts and Industries Building, which has stood on the Mall longer than almost anything but the Capitol.

The celebrants, led by secretary emeritus Ripley, basked in the glory of having developed something adventurous that, however amorphous in design, simply wouldn't stop working. As the Associates program exists now, it is more than just a service organization at the Smithsonian. It is an intrinsic leisure-time institution that has become an essential part of the community scene.

The figures are only one measure of the Associates' success. The program began with about 1,500 associates 20 years ago. Today it has 56,000 individual and family memberships -- representing 130,000 adults, young people and children in metropolitan Washington.

The programs stretch into all kinds of increasingly bold directions. Even Ripley could not have anticipated that the Smithsonian might launch a new "Singles' Evenings at the Castle" series, featuring champagne, canape's and lectures ranging from "New Directions in American Art" to "Forensic Anthropology."

Registration manager Marjorie Lee Walker is besieged by people who can't get into the course: "They call me and they tell me that I must understand how important it is to their lives."

The Associates' tours, once limited to fairly conventional visits to historical and cultural shrines, now serve members' leisure-time needs on a much broader scale. Tour coordinator Karen M. Gray has been amazed by the popularity of wintertime cross-country ski trips among middle-aged women. "I was away just before the inauguration last year when a trip was scheduled on a day the temperature was 10 degrees below zero. I would have canceled it. But 40 persons went. And they had tons of fun."

Among the other innovations of recent years are expanded programs for children; some scholarships for the needy; and the Discovery Theater series, a drama program serving about 60,000 in the recent season. Also, Solinger took on many of the Smithsonian's performing arts activities two years ago -- like the much esteemed Emerson Quartet concerts at the Renwick Gallery.

The heart and soul of the Associates' activities, however, have been and continue to be the courses, the lectures and the films, but their scope and size keep expanding to the outer limits of the liberal arts.

This fall's courses range from "Charles Dickens: Favorite of the Ages" to "America's Great Masters of Cooking: Trends in American Cuisine," with Julia Child, Craig Claiborne, Paul Prudhomme and Phyllis C. Richman among the lecturers.

The curriculum is directed at those who study "just for the pleasure of learning," said Solinger. "The majority are in their forties. Our most recent figures show that 40 percent come from Northern Virginia, 35 percent from Maryland and 25 percent from the District."

Even more basic is the fact that 88 percent of the associates have college educations. They are, as one of Solinger's deputies described them, "addictive learners . . . bookworms with joie de vivre."