If you knew Bidu, chances are you were at the Brazilian embassy until the small hours of yesterday morning, celebrating.

Bidu is Bidu Sayao, the elegant, ethereal Brazilian soprano who, along with Arminda Villa-Lobos, widow of the Brazilian composer, was the guest of honor at a party following Tuesday night's Inter-Amercian Music Festival Concert.

Bidu Sayao is not your run-of-the-mill elegant, ethereal soprano. Even today, 40 years after her Metropolitan Opera debut, 20 years after her retirement, knowledgeable folk still speak in tones of awe about her distinctive combination of voice and style, of her delicacy, her finesse. She is, said as fussy an authority as Rosa Ponselle in a telegram sent Tuesday, "the Queen of the lyric sopranos."

Add the factor of fierce national pride to these attributes of operatic legend and you get an idea of what went on at the embassy, where Sayao was hugged, kissed and photographed half to death while her over-awed admirers tried to put her qualities into some kind of perspective.

Harold Boxer, music director of Voice of America as well as general director of the Inter-American Festival, was starting out as a bass baritone at, the Met during Sayao's heyday, which ran from the mid-1930s into the mid-1950s. He remembers most "a certain tenderness a certain sweetness that was incorruptible. When she sang Mimi (in La Boheme) she was the perfect image of that innocence.

Others spoke of her diction, her purity, even her art, but no one was going to contradict Brazilian Ambassador Joro Pinheiro, who drew himself up into an official posture and said, "We consider her the Brazilian nightingale. She has been for more than 40 years our greatest asset abroad." Period.

The object of all this attention was a small, red-haired woman perched like a little bird on the edge of an ornate sofa. Just a week shy of her 75th birthday, Bidu Sayao said, "I feel very young inside of me," and she obviously meant it.

Something of the pixie marks Sayao, an animated quality, the ability to still tune into the wonder of things, whether it be a scheduled Thursday visit to the White House to meet Rosalynn Carter - "I'm thrilled to death about it," - or the decoration she received from the ambassador, the highest rank of the Order of Rio Branco: "They never give this to any woman, only men. I am the one expection."

Although the only things she sings now are "little Brazilian songs, just for fun" and never in public, and though she spends much of her time in a small town just north of Portland, Maine, Sayao finds that her fame is somehow increasing instead of shriveling away.

"I am very, very astonished," she says, her eyes widening even as she speaks. "All my fans are young people, I say, 'How can you remember me,' and they say, 'We know you, we love you from your records.'"

Perhaps her most famous record is the maddeningly haunting "Bachianas Brasileiras No. 5" by Villa-Lobos. "I was a very close friend to him, but he wrote this piece not for singing and cello but for violin and cello," she remembers. "So I told him, 'I want to sing that aria.' He had another conception, but I insisted." Gues who won.

To hear Sayao tell it, her enormous success in America was pretty much of an accident. "I came to America just for vacation," she says. "I knew Maestro Toscanini, I went to see him, he asked me if I could sing Debussy's La Demoiselle Elue." Nobody knew me then, but they were curious to hear this Brazilian soprano nobody knew about, so suddenly I start this career. I arrived at the right moment."

A right moment with almost a lifetime of preparation behind it. "I started when I was 13, I was too young, I had no voice, no hope, but sometimes this is better," she says. "I worked very hard from that age on. Some singers now need more perfection, more patience to study hard and not be in such a hurry to be a success. For me it was study, study, study, I sacrificed all.

"No," she says, stopping herself after a moment. "It was not sacrific really. It was a joy."