As a child, Aage Haugland was a soprano in the Copenhagen Boys Choir. "From what I have heard," he says, "I was a very ugly-sounding soprano, with a very, very high voice. Whenever they needed a high note I was called on. And the rest of the time I had to keep my mouth shut.

"Then my voice changed, and I became a very high tenor, which can be quite useful in certain oratorios. Any time they needed a high C, I was there. And the rest of the time I was not allowed to sing.

"Then at 17 I fell and hit my head and knocked myself out. When I woke up I was bass, again the most ugly sound you ever heard. But when they needed a low C, I was there. Now, I don't want to fall again, so I am very careful."

That was 22 years ago and Haugland hasn't knocked himself back to soprano yet. Instead, at age 39, he is becoming a major opera star--perhaps Denmark's finest gift to opera since the late Lauritz Melchior. Haugland's stardom reached a peak here last Wednesday when he sang Boris Godunov--arguably the mightiest role in the bass repertory--with the Metropolitan Opera for the first time. It was a Czar Boris of enormous poignancy and vocal beauty. Tonight at the Kennedy Center Opera House he will repeat it. Then, tomorrow night, he will repeat his delectable performance as the bumptious Baron Ochs in "Der Rosenkavalier," the role in which he made his Met debut in 1979. Haugland has been one of the most visible singers in what has turned out to be the most distinguished two weeks of opera that the Met has brought here in recent years.

Visibility has never been a problem, in fact, for the Danish bass. With his huge frame and dark blue eyes, he bears a strong resemblance to the young Charles Laughton, and people have been stopping him for years to ask if he is related. "I started introducing myself as the illegitimate son of Charles Laughton," he says.

At his hotel suite in Foggy Bottom he has pasted a large photograph of his daughter and son over the screen of the television set. He hasn't seen them, or his wife, since February. And although these months with the Met have been great for his career, they have taken a personal toll. "I will never accept anything like this again," he observes somberly.

There was supposed to be a reward when the four-month engagement ended. The family would join him, they would rent a camper and Haugland would decompress for a month touring the United States. But engagements have so piled up that his time with his family has been whittled down to 10 free days this summer at their vacation cottage at the Danish shore. Most of the rest of it will be spent singing at Bayreuth and Salzburg.

The night he sang "Boris" at the Kennedy Center, he says in British-accented English, "I felt happier than I have seldom felt in all my life. Would you believe that when I walked offstage, both the chorus and the stagehands were applauding as well as the audience. I have never experienced anything like that before.

"You feel very grateful, but you are extremely tired."

Only five days before his "Boris" debut he had sung Ochs on opening night here and, on the Met broadcast the previous Saturday, Klingsor in "Parsifal."

The role of Boris came his way about a month ago when bass Simon Estes withdrew from the part. Haugland still is not certain exactly who tapped him for Boris. "They knew that I knew it," he says. "I have sung all three versions there are wide textual discrepancies among them in four languages."

A complication was that the Met does "Boris" in Russian and that Haugland's most recent performance had been in English. Further, though Haugland is a bit of a linguist, Russian is not one of his languages, so he learned the role phonetically.

"That was the worst struggle in preparing the part," he said, "because it is very hard, on such short notice, to make the conversion of languages. For that reason, I wish I had had more time. My Russian was not perfect and even if there were only two persons in the audience who could tell the difference, that is enough for me to be concerned about it."

One reason Haugland's Boris is so moving is his rejection of the stereotyped czar, based on historical fact, as merely "a brutal, guilt-ridden maniac." Of course, Boris committed murder to reach the throne. But, observes Haugland, "It is essential to show a man whose guilt has cost him as much as the crime he has done. He realizes, for instance, that just as he caused the death of Dmitri, his own son will probably be killed immediately after Boris' own death. So he feels that he must give as much love as he can to his little son. There are two major scenes with the son. Both times Boris is burdened with guilt, which he tries to hide from his son. It is just as I would do with my own son if, let us say, I were having a heart attack, I would try to hide it, because it would frighten him. But then the attack goes away, I would turn back to him."

Asked if he believes if there are roots for this interpretation in the text, Haugland replies, "I hear it more in the music than in the words. I hear a very tragic soul in the music of Mussorgsky."

Haugland, whose father is a doctor of psychology at the University of Copenhagen, tried medical school for two years, but ran out of patience.

He started singing lessons, and to finance them, held two jobs. "I worked in the Tuborg brewery and simultaneously sang in a church. This combination is very good and I can only recommend it for future singers."

In 1968 he won a Danish competition. For the next two years he sang in the Norwegian Opera, then went to Bremen for three years and finally landed at Copenhagen's Royal Opera, where his repertory encompassed virtually the entire range of major roles for a low bass. His career began to expand with a Varlaam in "Boris" at La Fenice in Venice in 1975. And he made his American debut in 1978 with a concert "Boris" in St. Louis. At Bayreuth this summer he will sing Hagen in Solti's new production of the "Ring" and next fall he will sing Ochs with Solti at Covent Garden.

Haugland calls his family in Denmark twice a day. "By now the Bell System practically owns me. The calls may cost half of my fee," he observes. "But you have to keep in touch with some reality besides the opera. You know, the day before the first 'Boris' I called and my daughter asked, 'Daddy, are you nervous?' And I said 'yes.' And then instead of consoling me, she changed the subject and said, 'Daddy, I am going to ride a new horse today.' That was what I needed."