For a moment, it was like the old days: a little five-person troupe in the funky upstairs of a famous Greenwich Village club, putting the Republican establishment to shame with satire and song.

"Rap Master, Ronnie," written by Doonesbury creator Garry Trudeau, opened tonight at the Village Gate and poked sometimes savage fun at the Reagan administration on everything from Grenada to cheese give-aways to nuclear deterrents.

There was a president -- given to cowboy hat -- with diction eerily similar to that of the man in the White House -- as well as a penchant for naps. ("From 9 to 12 this cowboy earns his keep," went one satirical verse. "But counting Commies makes a cowboy tuckered, so 12 to 5 this old cowboy's countin' sheep.")

There was a smattering of celebrities in the audience: actress Joanne Woodward; Marion Javits, wife of the former New York senator; Eleanor Mondale, daughter of the Democratic presidential nominee; writer Kurt Vonnegut and his wife, photographer Jill Krementz; cartoonist Jules Feiffer and his wife, writer Jennifer Allen.

Not surprisingly, when the show was over, at least one member of the audience was crazy about it.

"I thought it was great," said Eleanor Mondale. "It really brought out a lot of the inconsistencies in this administration . . . I think everybody should fly up from Washington to see it."

Feiffer, with a few decades and a few elections on Mondale, took a more cautionary approach.

"I thought it was charming," he said, "I thought it was funny and I'm afraid it won't elect Mondale."

The new show, created by composer Elizabeth Swados and Trudeau, whose Pulitzer Prize-winning strip Doonesbury returned to the papers Sunday after a two-year absence, grew out of the Broadway version of Doonesbury. In that show, a Reaganesque president often spoke to the audience in a jocular yet calm voice, and in the last weeks of the show its creators wrote the president "a rap song" as a bid for minority voters.

The rap so impressed a record producer that he persuaded Swados and Trudeau to expand it. In July, Newsweek called it "hilarious," and tonight Swados confided that in some parts of the country it might have even been construed as a hit. "I think we made the charts in Cleveland," she said.

Was it the sort of thing that people were running around humming?

"That's hard with a rap song," said Swados.

There was no comment from Trudeau, traditionally a notoriously press-shy fellow.

There was, however, in this evening's entertainment, the lovely ear -- and bite -- that has marked Trudeau's strip for years. His president, played by Reathel Bean, appears flanked by a dancing chorus of Secret Service men in shades and gray flannels -- there's a lot of gray flannel in this spoof on Republicans, of course -- and begin with his rap for the minority vote.

"Okay, people, ease on through, Rappin' Ron Reagan got cheese for you," he chants, and, as the song ends, "Light my fire, black people . . . Papa's got a brand new safety net for you . . ."