Despite WETA's ups and downs, last night's gala dinner to celebrate its 20th anniversary was as elegant and smooth as a perfect airwave.

The evening at the Pension Building, complete with Gene Donati's big band, dancing and potted palms, was sooo elegant that comedian Mark Russell couldn't resist the quip: "Channel 26 must have had a better membership week than we thought."

But just as WETA celebrated its 20 years on the air, the convoluted history of public broadcasting took yet another turn. Faced with sharp cutbacks in the federal budget and unsure of the financial alternatives, a special commission resolved this week to experiment with limited advertising on a few public radio and TV stations.

That authorization, by the Temporary Commission on Alternative Financing for Public Telecommunications, said the experiment shall extend to no more than 10 TV stations and 10 radio stations.

WETA will not be among those, said its president Ward Chamberlin. "It's not appropriate for a UHF station in the nation's capital." Of the proposal for commercials, Chamberlin said: "We need funds, but what's the point if it will change the nature of public television?"

PBS president Larry Grossman agreed. "I can't argue with it on an experimental basis. But I think it has all kinds of dangers . . . I'm skeptical about people's attitudes and the effect it would have on public support."

The chairman of the House communications subcommittee that initiated the study this summer, Rep. Timothy Wirth (D-Colo.) was among last night's guests and indicated he was pleased with the decision. In response to the fear that commercials may change public TV he said, "public broadcasting is ever-changing and should always be experimental."

But in spite of the weighty consideration of the evening, everyone was in the mood for celebration. Comedian Russell, one of the "special guests," was in superior form last night:

"It was either this," he said of the party, "or give Jim Lehrer a new contract."

"If Channel 26's event looks like this, Channel 9's must be in the Taj Mahal."

"This must have been what Reagan meant when he said he was going to put life in the ghetto."

And, irresistibly:

"Have you seen Misterrogers yet? I understand he's had three Shirley Temples."

Fred Rogers was there, but he said he'd had "only one," and that he was hoarse from trying to talk over the noise of the band and more than 700 excited people. "I've been screaming; I've never been to such a loud thing," he said, as quietly and gently as he does on TV. "People cannot believe that I'm real. That's not my fault. But those who know me know I'm real."

For many, one of the most amusing things about the evening was meeting familiar faces from the screen and discovering that they are, in fact, real. Julia Child and Mary Martin were overwhelmed by guests trying to shake their hands, get autographs and exchange a few words. "People in Washington are always dying to meet the movie stars," said an observing Cokie Roberts (of "The Lawmakers"). "And the movie and TV stars always want to meet the ones in Washington."

"You're my idol," a young woman said to a twinkling Mary Martin.

"Was 'Peter Pan' the first thing you ever saw?" Martin replied. "I can always tell someone's age by what they remember," she said when the woman nodded yes.

And a towering Julia Child in a blue tunic displayed a well-seasoned attitude toward the attention. "It's part of the business, you know. If you don't like it, too bad for you."

Among the others who turned out to wish WETA well were, of course, Elizabeth Campbell, its founder, who said "I feel numb"; Aaron Goldman, chairman of the board of WETA; former mayor Walter Washington; Georgetown University's president,Rev. Timothy S. Healy; Livingston Biddle, chairman of the National Endowment for the Arts; "Upstairs, Downstairs" creator Jean Marsh; Linda Wertheimer and Jim Lehrer.