Placido Domingo, at 6-foot-2, stands bare-chested before the full-length mirror of a drafty little fitting room, amid the massive disarray of the Metropolitan Opera's cavernous costume department.

He has just finished squeezing into the five glittering and cumbersome 16th-century Spanish outfits he will don this season in the title role of Verdi's "Don Carlo." He awaits the fitting of two mid-18th century Parisian waistcoats he will need as the passionate Maurizio, count of Saxony, in Cilea's "Adriana Lecouvreur." He jokes about the effort.

"Twelve years ago I didn't have to do all this fitting," he says. "I was so big they made special costumes just for me."

A white frock coat is donned and discarded. "Even though I lost all the weight, on stage you still look bigger in white." He asks the fitter, "Can't you spray it gray?"

And as he gets in and out of the silks and surplices and tights and tunics and brocades and capes that help make opera the most spectacular of the performing arts, he rails about the inequities imposed upon him by his sex. "If everything fits down to the waist for the ladies, it doesn't matter much below," he mutters. "Because of the huge skirts."

A sudden flurry of babbling voices is heard in the corridor outside. Domingo looks startled: "It is a tour group!" and there is a rush to close the curtain before the two dozen visitors, most of them elderly women, can glimpse the tenor. He sighs with relief.

A few minutes later, dressed in an expensive-looking checked suit (Bellini of Milano), Domingo ventures out. As he nears his dressing room, he walks right into the same tour group.

The tourists look awed and at first seem uncertain what to do. Finally, there is a little rustle of applause. One woman rushes forward. "They told us we couldn't speak with you, Mr. Domingo," she blurts. "And I understand that. But maybe I could just shake your hand." Domingo beams, extends his palm, and disappears into his dressing room.

ENRICO CARUSO once memorably described the common stereotype of the tenor. It consists, he said, of "a big chest, a big mouth, 90 percent memory, 10 percent intelligence, lots of hard work and something in the heart."

Placido Domingo, like Caruso, fits some of that description, but not all. Dumb he's not. Temperamental he's not. In the way he has conducted his career, in his offstage manner, he seems... well... normal.

And with the remarkable predictability of his vocal excellence over 22 years of performing, he also flies in the face of another common perception of opera singers -- the cliffhanging drama of the glory of their high moments and the disaster of their lows.

For that barrel chest reflected in the mirror powers one of the protean voices of our time -- or of any time. With his principal rival, Luciano Pavarotti, in apparent vocal decline, Domingo's is arguably the most distinguished opera career of our time. He is scheduled to sing a concert at the Kennedy Center tomorrow night, his first public appearance in this city in 11 years.

He cuts a striking figure physically, with his dark Latin looks, his flowing black hair, his large features and especially his big, intense brown eyes. He is courteous, cooperative, gregarious. Everyone at the Met, from music director James Levine to the seamstresses in the costume department, addresses him as "Placido." He has decided opinions on opera ("There is nothing more beautiful than a great performance and there is nothing more boring than a really bad one") and sports ("I still cannot understand why you do not have a baseball team in Washington... Well, you are in seventh could now with the football team. Yes?"

In a profession that gave the term "prima donna" its pejorative meaning, Domingo, 42, seems calm and self-possessed. He is a workaholic, with a total of 1,582 performances to his credit as of last spring, and he has an astonishing repertory of close to 100 roles. He has made roughly 50 complete opera recordings, including three "Aidas" and a third "Carmen" that has been taped but not yet released. There are also three Verdi Requiems. The late diva Rosa Ponselle wrote of him: "Placido's promise, even at the height of his career, is immense, and he undertakes new roles with the frequency and rigor of Caruso."

Domingo insists, though, that he does not function at this pace compulsively. Too much emphasis has been put, he says, on "what they call this absolutely incredible frenetic career. People have been saying from the very first year that if I keep it up I will be voiceless, that in no time I will no longer be able to sing." Years ago he took up the title role of Verdi's "Otello," perhaps the most strenuous single tenor role in Italian opera, and now he is almost indisputably the leading Otello of the day. "They kept saying, 'Otello is going to kill you,'" Domingo remarks, "but people make statements like that just so they can say, 'You see, I was right all along,' in case it's happening."

But it hasn't happened to Domingo. At an age when many singers are beginning to have to hedge their vocal bets, his voice is still developing. "I am finding this year at the Met that there are several things that I am able to do that I never could quite do before," he says.

Domingo has pulled off the rare feat of mastering heroic roles like Otello, Calaf in Puccini's "Turandot" or Wagner's Lohengrin -- with the enormous volume and sonority that they demand -- without sacrificing the sweetness and delicacy of sound needed for Rodolfo in "La Boheme," for instance. He was especially proud of the Rodolfo he had sung the night before at the Met. "It went just about as well as it has ever gone," he said. "I feel that when I can sing it better and fresher than ever at the age of 42, I know that I have been doing something right, you know."

The natural sound of Domingo's voice would seem to be a sort of bronze timbre, resonant but also sharply focused. Sometimes it is close to the sound of a baritone. "I always think of the cello as an inspiration," he says. "The cello for me is the most beautiful instrument. I like to imitate the sound of the cello, and I have a very beautiful compliment from Rostropovich because he said, 'No cello sounds like Placido.'"

One of the keys to his versatility is a remarkable ability to color the voice differently depending on the needs of a particular work. "Color is the basic thing," he observes. "One day I read the comment of a singer that color doesn't count, what counts is only the voice. You sing with your voice and it never changes. I believe that for me it is exactly the opposite. Even in a very dramatic opera like 'Otello' the love duest has to be colored with such a lyric color and then you have to change the voice for the more dramatic moments. But I still think that the singing that most touches the heart is the singing that is very warm and legato."

Even so, he estimates the average extent to which he matches his voice to what is in his head "at only about 30 percent, which is really pretty good." And he is a harsh self-critic. Commenting on his critically acclaimed performance on a recently released recording of "Turandot," he says, "I had to sacrifice a little on the B-natural at the end [of the aria 'Nessun dorma']. It doesn't..." and he slaps his hands searching for the right word, "it doesn't turn the way I would like the note to do."

It is rare for a singer with Domingo's broad command of Italian opera to venture into the heavier Wagnerian roles, for fear of vocal damage, but any tenor who can handle Otello can handle many of those parts. Domingo is moving cautiously in that direction. He recorded a superb performance of Walther in "Meistersinger" several years ago. There is the upcoming "Lohengrin." And, most ambitious of all, there is the possibility of a "Tristan and Isolde," which Domingo describes as "together with Otello the ne plus ultra of the repertory for the tenor."

Asked if he will sing it, he replies, "There is a certain temptation. A great temptation, in fact. But I have to think carefully about it... I have the feeling that in order to do Tristan I will have to do something very peculiar.I think that for Tristan I would really stop for six months to prepare the part... I might learn the part completely and then sing the third act in concert. I do believe that if I can manage the third act the rest is no problem."

DOMINGO IS Spanish, but the demands of opera companies around the world have made him into a classic example of the modern jet-setting artist. Ask him where his home is and he answers, "Barcelona, London and New York." But there is also Milan for La Scala and Salzburg in the summer and dozens of other places in between. Barcelona is home base for his wife, Marta, and his three children.

Domingo has traveled across the continents since his childhood. He was born in Madrid but grew up in Mexico City, after he moved there with his parents, who were noted singers in the Spanish operetta form called the zarzuela.

He began to study the piano at 6 and his parents deliberately did not encourage a singing career. He was 20 before making his operatic debut, as Alfredo in Verdi's "La Traviata" at Monterrey, Mexico.

Most singers develop their voices first and then do their musical training.Domingo came the other way around. "If I have a bigger capacity for work than most singers," he says, "it is because of that. I do not have to go to a coach. I play the piano, so I teach myself.

"I am very lucky for the fact of being so interested in music itself, because of my parents' putting me to study piano seriously. It gave me a great advantage, which is that I don't have to worry about the music most of the time. Once I learn it, you know, it's so much inside of me that I am part of that music. I study very deeply the scores. I study the orchestration. I study the way to blend my voice with the orchestra. I try to immitate the instrumentation of the music in certain moments.

"I determined from the very beginning to be realistic. I said, I can do musical comedy, I can do movies, I can sing popular songs [such as in his recent hit record with John Denver, "Perhaps Love"], I can do zarzuela, I can conduct [which he has been doing for a number of years]. So I decided if my career was going to be in opera I have to go right to the very top, because this is a world that fascinates me in an incredible way because of all the possibilities it has, because of all the combinations of music, acting, dancing, scenery, costumes and so on. So I had to put a limitation on myself. I decided that at 30 I would be already established in the main theaters or I would give up and do other things. Because I thought opera was so important that I would only enjoy it on that level. Because I would rather be the head of a mouse than the tail of a lion."

Only twice, says Domingo, has he run into some of the vocal problems that plague many singers."Once was in 1968 and the other time was in 1978," he says. "There were difficulties in the transitions from range to range, and also I found that I was pushing my voice. So I lightened my work.

"My average of cancellation is about one performance every three years, or two every four. Only two times in my life have I given up after an act. One was in Hamburg in about '72 or '74 in a 'Forza.' The other was this fall at the Met."

That night at the Met, he left because of illiness after the first act of "Gioconda," spawning a near riot in the audience. The feckless stand-in, a relative unknown named Carlo Bini, was booed from the moment he came on stage. The conductor finally had to stop the music and reprimand the audience.

Though the Met does not pay the fees that Domingo can command elsewhere, he has been a mainstay there since his debut in 1968 in "Adriana Lecouvreur." "I am a fanatic of the Met," he declares. "I come every year and I do a lot of performances." He speaks scornfully of singers who visit the Met just occasionally "only to keep their names on the roster."

In recent years Domingo has skipped the annual Met spring tours. But because next season is the Met's centennial, he will go on the tour. He plans to sing three times during the Met's two weeks at the Kennedy Center -- two performances of a revival of Zandonai's "Francesca da Rimini" and a "Tosca."

He mentions another possibility for Washington, an opera being written for him by Gian Carlo Menotti on the life of Goya, which the Washington Opera "would like to premiere." He says he has not seen the music, "but people tell me it is beautiful."

ONE OF THE most visible consequences of superstardom for Domingo has been the heated rivalry between him and Pavarotti. It is unclear who cast the first stone, but the conflict was rendered intense by such events as the PBS broadcast of "La Gioconda" from San Francisco several years ago. Renata Scotto become so enraged at the way she thought Pavarotti was hogging curtain calls that, before the live camera, she blurted, "Placido Domingo is the greatest tenor in the world!"

And things were not helped any last fall when Pavarotti declared to a Playboy magazine interviewer that Domingo's album with John Denver was "in bad taste." And he also maintained that Domingo's presence in a television show dedicated to Caruso was "unfair" and "very embarrassing."

Domingo refuses to reply in kind. "I hope the whole thing is dying out," he says. "You know, everything has a crescendo and a decrescendo. In a way it was a logical thing to have happen, because it's happening everywhere in life, in sports, in politics, and so on. I don't like to talk about it. I think he is a great artist, a great singer, you know.

"If you are doing an article where you are feeling more sympathy for him, you don't need to put me down, or if you have sympathy for me, you don't need to put him down. And, you know, some of these people who are putting him down, maybe they were praising him just a month earlier. And the same will happen with me probably. So I like people to be a little more delicate in their comments."

In ways the Domingo-Pavarotti comparison is inapt anyway. Pavarotti's voice is a fairly light lyric tenor of the bel canto variety. It is his ventures into the heavier roles in "Gioconda" and "Aida" that seemed to have taken a premature toll on that lyric sound, which at its peak was sometimes even more distinctive than Domingo's in the same repertory. This season the 47-year-old Pavarotti has canceled more American dates than he has sung, including all his San Francisco and Chicago appearances, and many of the ones at the Met.

Given Domingo's history of vocal stability and stamina, there is no reason why his voice cannot remain at its present peak for quite some time. It would be surprising, in fact, if it did not.

He is making no precise predictions, but he does say, "I hope to work in opera to my 25th anniversary at the Met, which would be 1993. I would be 52. But I don't know if I will go until I am 60. You know, you also have to think about how you look on a stage. I do believe in theater that is believable. And, of course, if you still have the voice after that, you can do concerts."

He says he is like "those three pitchers who are all trying to get 300 wins -- Seaver, Palmer and Carlton. I hope they all make it." Carlton actually has made it.

And as to that inevitable day when he will stop, he says, "It will be sad. But it won't be a tragedy, as it is for so many singers. There are too many other things for me to do in music."