"This was my operatic debut," said Jean Stapleton, excited as a debutante. "I started at the top, and the only way I can go is down." The actress was relaxing at a party given by The Watergate in its Terrace Restaurant after last night's gala performance of "The Washington Opera Follies of 1983." An hour earlier, next door in the Kennedy Center's Terrace Theater, as part of the benefit for the opera company, she had given the first Gilbert and Sullivan performance of her career, in tandem with D'Oyly Carte veteran John Reed, earning the wildest applause of an applause-ridden evening.

Across the room, on the dance floor, Reed was enjoying himself with a Washington Opera staff member, vibrating to the very un-Gilbert and Sullivan-like sounds of "You Ain't Nothin' But a Hound Dog," played by the band. A few minutes later, he was in a quiet corner, discussing fine points of technique with a fellow connoisseur of knitting machines, then reflecting on how conductor Cal Stewart Kellogg had decided that he didn't need to rehearse all the verses of the nightmare song from "Iolanthe" ("When you're lying awake with a dreadful headache . . . "). "He was probably right," he said philosophically. "The more you do that thing, the worse it gets. But the people did applaud a lot, didn't they?" Indeed, they did--500 of them who had paid $250 per ticket for a variety show that ranged from Catherine Malfitano of the Metropolitan Opera singing a torch song to Mstislav Rostropovich playing Tchaikovsky, Britten and Popper on his cello.

Not all of the stars who had made the Follies a true gala turned up for the after-show party. Ethel Merman was not at The Watergate, but she had left a hospital bed, suffering from sciatica, to come down to the Kennedy Center and close the program with "There's No Business Like Show Business." "When she obligates herself to do something, it is ironclad," said her manager, Bob Gardiner, who did make it to the party. "Marty Feinstein called me a couple of days ago and asked me, 'Do you think she'll make it?' and I told him, 'She has no intention of not being there.' "

"We actually rehearsed our number in the Roosevelt Hospital in New York," said Jerome Hines, who stepped out of his Metropolitan Opera persona to do a duet with Merman on "You're Just in Love," from "Call Me Madam." "She told me on the phone, 'You're tall and I'm small, so it should be very funny when you put your head on my shoulder.' " She was right. Hines, who must be at least two feet taller than Merman, went through some incredible contortions to get his head on her shoulder during the song, and the audience went wild. At the party, he was sitting in a quiet corner, drinking toasts with his wife, white wine in her glass, ice water in his; Hines is a devotee of health foods and a walking advertisement for them. He was thinking deep mathematical thoughts, preparing for his next book, which will be a popular treatise on mathematics, "Stalking the Elusive Glitch." His first book, a series of interviews with colleagues called "Great Singers on Great Singing," already has sold out its first printing, he announced happily. The second printing will contain the chapter on Renata Scotto which accidentally was left out of the first.

At the next table, another married couple also was sitting quietly--Tammy Grimes and her husband, pianist Richard Bell, who appeared together for the first time as husband and wife during the Follies. "I had trouble reaching her to make final arrangements for her appearance," recalled Martin Feinstein, general director of the Washington Opera. "Finally, on Friday, I reached her agent and he told me, 'She's on her honeymoon, but she'll be arriving at 2 this afternoon.' "

A romantic air pervaded the party. Bass Franc,ois Loup, who has played in three of the Washington Opera's productions this season and is discussing roles for the next three seasons, was escorting his fiance'e, soprano Mary Beth Parrotta, whom he met during the Washington Opera's "Tosca," while she was singing in the chorus and he was singing the role of the Sacristan. "I will be back in June to marry her," said Loup, whose current engagement with the company ends on Sunday. "Then, we will go to live in Cannes. I hope to sing 'Don Giovanni' with her in France; I will be the don and she will be Zerlina."

With Stapleton and Reed, too, "It was love at first sight," according to the British expert on Gilbert and Sullivan, but its was not as romantic as in the case of Grimes and Bell or Loup and Parrotta. "We met on Thursday, and we're old friends now. We've exchanged numbers and I'll see her and her family when I'm in Los Angeles singing 'Mikado' with Roger Wagner. She's the sort of person you want to see again."

She is, in fact, the sort of person you want to see again in Gilbert and Sullivan. Her voice is not quite equal to that of Marilyn Horne, but it has a power and expressiveness that more than compensate for an occasional, microscopic inaccuracy of pitch, and she mimes and dances with a unique comic flair. In the solo "Sad Is a Woman's Lot" from "Patience," she walked onstage with a cello, could not get it to sit comfortably between her knees, tried standing it upright on the floor and ended up putting it on her chair and standing beside it. She drew the bow across the strings and looked amazed when a sound emerged. A minute later, she missed a cue, forgot to move the bow, and looked (quite rightly) even more amazed when the sound came out anyway, drifting up from the orchestra pit. Her singing is comparable to most mezzos heard in Gilbert and Sullivan productions, and her comic talent is much larger. Judging by her enthusiasm after last night's performance, Martin Feinstein probably can hire her for opera at a fraction of what it would cost to have her simply talking.