Reporters were not allowed to witness the swearing-in this week of Frank Zampatori as a D.C. Alcoholic Beverage Control Board commissioner in the mayor's office. ''You're not always allowed access to the Oval Office, you know,'' city Communications Director Ed Meyers offered by way of explanation.

But the ceremony, performed by Mayor Marion Barry, was witnessed by some of the city's most prominent homosexual activists. Afterward, they were wearing Cheshire Cat smiles.

Zampatori is gay, the ABC board is one of the city's more powerful agencies, and the gay activists could hardly be happier at the appointment and what they see as increased attention from the mayor and his staff.

"Your phone calls get answered," said Andy Hirsch, vice president of the Gay Activists Alliance. "Before, you would talk to a staff member about something and then you'd talk to the mayor, and he wouldn't always be aware of what you were talking about. Now you can be sure he knows."

Gay organizations like the Gertrude Stein Democratic Club were important to Barry in his 1978 campaign. Homosexuals may not be a powerful voting bloc in Washington except in Ward 2 (which includes Dupont Circle), but they have proved to be one of the most effective special-interest groups in town. They were effective campaign workers for Barry and generally have been among his most loyal constituents.

As campaign time rolls around again, Barry wants to be able to count on an army of workers to distribute leaflets, tack up signs, staff phone banks and perform the many tasks that keep a campaign organization running. Zampatori's appointment demonstrates just how much Barry wants to keep the gays' support. It appears he's on the right track.

In a "midterm report" on the Barry Administration at the beginning of the year, the gay alliance said it was pleased with the mayor's appointment of acknowledged homosexuals to a number of D.C. boards and commissions. But the group said it wanted more gays to be appointed to panels with power, not just to the strictly advisory ones.

The ABC board, with its control over liquor licenses, is one the gay community has had its eyes on. "This is very significant for the gay community," Zampatori said moments before the swearing-in. "We have 32 (gay-oriented) restaurants and bars in the city. We haven't had many problems recently, but in the past, there have been problems."

Paul Kuntzler, a founder of the Stein Club, said that, in general, the District's ABC board has been good as far as gay establishments are concerned. But he said local gays remember the days when police conducted raids on gay bars, and are mindful of problems gay establishments have in obtaining liquor licenses in other cities around the country.

The appointment, said GAA President Melvin Boozer, "certainly won't hurt" Barry in the gay community. "In the past few months, we've been much more involved in mainstream things than ever before -- advisory panels, things like that. It's just been a tremendous difference."

As he said that, Boozer was wearing one of those Cheshire grins. Barry is not the only mayoral hopeful lining up to get on the gays' good side. Former City Council chairman Sterling Tucker, for example, spoke to a gay alliance meeting recently. Barry was scheduled to address one of the organization's meetings Tuesday night.

The area in which gays see most progress, said Boozer, is in their relationship with the police department. They distrusted former police chief Burtell M. Jefferson and he distrusted them. One of the demands of their midterm report was that Jefferson make a clear public statement that his department does not discriminate against homosexuals. Jefferson did not reply.

But the gays say they see a world of difference in the new chief, Maurice T. Turner, and view his appointment as one of the most important things Barry has done. "He's fantastic," Hirsch said of Turner. "I can't say enough good things about him."

Turner, unlike Jefferson, has been willing to talk to the gays about police-related situations on which they want action. For example, Hirsch said, Turner has asked his officers to work with gay activists on a series of murders of young black men that they think might be gay-related. There is also increased cooperation between the police and a gay-oriented telephone "crime line."

And Turner has met with the commander of the Marine Barracks in Southeast Washington to discuss the series of confrontations between gays and Marines that have taken place in the vicinity of Equus, a Capitol Hill gay bar. The situation was a sticky point between the gays and the police, and the gays now believe it to be under control.

Barry has been criticized in the past, particularly by some of the city's black ministers, for his support of gay issues. But his support continues. The gays' midterm analysis was largely favorable; and they now say they've already gotten most of the things they wanted from the final two years of Barry's term.