"TO MOST AMERICANS, the Met is opera," Nancy Reagan said a week ago in a White House ceremony honoring the Metropolitan Opera. It may well have been the least controversial statement any member of the first family has made in years.

The Metropolitan Opera, which opens its 100th-anniversary season tomorrow night, is the world's largest performing arts institution, with 2,000 on its payroll. It is also the most expensive, with a budget of $70 million. Even the physical dimensions of the Met, which is the centerpiece of Lincoln Center since moving there from its old headquarters at Broadway and 39th Street in 1966, are remarkable. It is not tall, but it covers a territory equivalent to a 45-story skyscraper turned on its side. The Kennedy Center is, by comparison, a mere babe.

Further, and very much to the point, the Met is not just a New York opera company: it is the national and international showcase, as well as the progenitor, of a thriving American operatic community. Through the spring tours it has made for most of its history, through its half century of weekly Saturday afternoon broadcasts and its regular opera telecasts, the Met has brought the noblest of the arts to more people than any other institution in history. It is now molding its sixth generation of listeners.

A brilliant galaxy of American opera singers has developed in the last 60 years, in a field that was once the exclusive province of the Europeans. It is estimated that there are now more than 1,000 organizations in the United States presenting opera in some form, including about 50 with budgets of $500,000 or more. It probably would never have happened--certainly not on this scale--without the example, the inspiration, the pervasive presence of the Met. It has built a national audience that has proceeded to build other opera companies. And now, for the first time in its history, the Metropolitan Opera's leadership is in entirely--and extraordinarily gifted--American hands.

Tomorrow night, artistic director James Levine will raise his baton in Lincoln Center for the opening notes of Hector Berlioz's "Les Troyens." Levine, who is 40, is passing out of the boy-wonder phase of a career in which the sky seemed to be the limit, and since his appointment as the Met's music director in 1975 he has been reshaping the venerable opera company.

As it rounds out its first century, the Metropolitan is the world's most prestigious opera company--partly because of current economic problems in Milan and Vienna. That prestige is evident in a simple list of those who will perform Oct. 22 in a marathon gala program marking the actual day when, in 1883, the Met raised the curtain on its first production, Gounod's "Faust."

They range alphabetically from John Alexander to Frederica von Stade. There will be approximately 75 international stars on the stage during the 10 to 12 hours of the gala, including Martina Arroyo, Carlo Bergonzi, Leonard Bernstein, Grace Bumbry, Montserrat Caballe', Jose' Carreras, Ileana Cotrubas, Re'gine Crespin, Placido Domingo, Simon Estes, Mirella Freni, Nicolai Gedda, Nicolai Ghiaurov, Jerome Hines, Alfredo Kraus, Evelyn Lear, James McCracken, Robert Merrill, Sherrill Milnes, Birgit Nilsson, Jessye Norman, Luciano Pavarotti, Leontyne Price, Regina Resnik, Elisabeth So derstro m, Teresa Stratas, Joan Sutherland, Kiri Te Kanawa, Jess Thomas, Tatiana Troyanos and Jon Vickers. It would almost be easier to list those who will not be performing--but that is another story. The gala has monopolized the evening for so many opera stars that at least one other company--San Francisco--has chosen not to perform that night.

Paradoxically, as its age moves into three figures, the Met is also, in many ways, the youngest, most versatile and least tradition-bound of the world's international-level opera companies. Its annual budget is derived almost entirely from ticket sales, endowments and private contributions, without the kind of heavy government subsidy that once made life easy for European opera companies. That helps a lot, but the key element is the guiding presence of James Levine.

His reshaping of the Met has many dimensions, including a broadening of repertoire, increased emphasis on theatrical and visual values, systematic encouragement of young American singers and, in general, a greater willingness to take risks. The likelihood is that this process will continue--and accelerate--in the same direction. This month the Met announced Levine's promotion from music director to artistic director. In many ways, the announcement simply formalized a fait accompli; Levine's role at the Met has long been broader than that of a music director, choosing repertoire and performers and conducting a high percentage of performances. But the new title will consolidate his role and give him almost absolute control of every artistic aspect of the Metropolitan's work, from the repertoire to the assignment of stage directors and set and costume designers. No other musician in history has had such power at the Met--not even Gustav Mahler or Arturo Toscanini.

What Levine's new title really means is that the artists are taking the direction of the company away from the money people who started it a century ago and who have made most of the key, long-term decisions since then. If all goes well (and things usually go well for Levine), the decades ahead at the Metropolitan Opera should show a more consistent artistic philosophy in the Met's activities, more coherent patterns, and the impact of a single, highly developed and wide-ranging taste.

It has not always been this way. The Met has had more than its share of what it is now fashionable to call "life-threatening" crises.

To mention just a few: The fire of the 1892-93 season, which almost ended the Met before it was really established. The financial disaster of 1933, in which the company's reserves were wiped out by the Depression, after almost a quarter-century of luxury when the deficit had always been paid off by the Met's backers, especially financier Otto Kahn. The labor crisis of the 1961-62 season--the most dramatic of many--when general manager Sir Rudolf Bing canceled the season, only to have it saved at the last minute by the personal intervention of President Kennedy and Labor Secretary Arthur Goldberg. The funding crisis of the 1970s, when people began to fear that opera had become so expensive that it was as doomed as the dinosaur. In that crisis, the Met was rescued by a commitment to modern management techniques that has put the company in perhaps the best state in its history.

There were also the moments of unforgettable personal drama possible only in the arts medium that packs the greatest emotional intensity. There was December 24, 1920, the last performance of Enrico Caruso, the tenor who arrived in 1903 and sang every opening night but one until his death in 1921. His body was so riddled with cancer that he could barely stand. But by every account, the performance, of Hale'vy's "La Juive," was magnificent. Apparently Caruso knew that this performance was the end, and so did many in the audience. Caruso, through his recordings, had almost single-handedly taken the company and made it a household word in the western world. Even today, Caruso probably remains the artist who had the single most sensational vocal career in the Met's history.

A similar moment came on March 4, 1960. The opera was, ironically, Verdi's "La Forza del Destino." Renata Tebaldi was Leonora. Richard Tucker was Don Alvaro and Leonard Warren was Don Carlo. In the second act, right after the words "Oh, gioia, oh gioia," Warren, who was the grandest of the whole generation of Americans introduced during World War II at the Met because Europeans were in short supply, pitched forward in what appeared to be a dramatic bit of stage business. But conductor Thomas Schippers knew better. He stopped the orchestra, and the curtain was lowered.

For more than an hour members of the audience waited in their seats or in the lobbies. Finally Rudolf Bing appeared before the audience.

"This," he said, "is one of the saddest nights in the history of the Metropolitan," bringing outcries of "No!"

"May I ask you all to rise," he went on, "in memory of one of our greatest performers, who died in the middle of one of his greatest performances."

Then he added, "I am sure you will agree with me that it would not be possible to continue with the performance." And the listeners filed out in shock.

There were also breathtaking triumphs. Just consider this lineup on January, 23, 1908, for Mozart's "Don Giovanni," one that could not even remotely be matched today. The title role was sung by Antonio Scotti, Leporello was Feodor Chaliapin, Donna Anna was Emma Eames, Donna Elvira was Johanna Gadski and Zerlina was Marcella Sembrich. The conductor? Gustav Mahler.

Another milestone in the Met's emergence as a major center of opera came on December 10, 1910, when the company presented its first world premiere, Puccini's "La Fanciulla del West" ("The Girl of the Golden West") with Toscanini in the pit, Emmy Destinn as the cowgirl Minnie, Caruso as Johnson and Pasquale Amato as Jack Rance. Puccini and David Belasco, on whose play it was based, supervised. It failed to gain a foothold in the repertory but in the 1960s it was revived with Leontyne Price and now seems to be gaining its place.

There is also the story of a brilliant surprise replacement who changed the course of the singing of Wagner. The celebrated soprano Frida Leider decided in the summer of 1934 that alterations in the exchange rate made coming to the United States for the next season impractical, so she canceled, leaving the Met in the lurch.

Leider must have been laughing all the way to the bank when she heard that a singer named Anny Konetzni, who replaced her as Bru nnhilde, had failed.

But her glee would have been short lived. On a broadcast performance of February 2, 1935, there was the Met debut (as Sieglinde in "Die Walku re") of a singer almost 40 who was little known out of her native Norway, on whom the prelimininary reports had been modest and who was already contemplating retirement, feeling like a failure. She was Kirsten Flagstad.

As Irving Kolodin describes it in his book, "The Metropolitan Opera," after that afternoon "other heroic sopranos were something like excess baggage." Leider was quickly forgotten. And the glorious Flagstad and her eminent tenor partner Lauritz Melchior launched the Metropolitan Opera on a golden age of Wagner. They became figures as towering in Wagner as Caruso had once been in Italian opera.

Caruso and Warren and Price and Flagstad were just four of the most conspicuous singers who reached international attention primarily through the Met.

The company's record as a star-maker, however, has not been perfect. There are at least three sopranos of modern times whose careers at the Met were botched--Elisabeth Schwarzkopf, Beverly Sills and Maria Callas. And in each case the villain seems to have been Rudolf Bing.

Schwarzkopf became one of the greatest of singers soon after World War II, making almost yearly appearances in this country, but she did not appear at the Met until the 1960s and, as she remarked last year, "I was not singing at my best at that time." The Sills case was even more inexplicable, because she was the star of the New York City Opera, right across Lincoln Center plaza from the Met. When she was finally invited over, Sills, who is now the executive director of the City Opera, was treated royally but her voice was not as reliable as it had been.

Bing's stormy relations with Callas are the stuff of legend, and there is no question that she was a very difficult person ("a black orchid," one person dubbed her). But she was the most important singer of her time. And she was, in fact, born in New York. So it would seem that the general manager would have felt obligated to present her there regularly, regardless of the problems. She made her Met debut in the 1956-57 season, and within two years she and Bing were at each other's throats over his insistence that she do Lady Macbeth and Violetta in "Traviata" within eight days of each other.

She was matchless in both roles, but she complained, "My voice is not an elevator going up and down." He became so enraged that he dismissed her and added, for no particular reason, "Let us all be grateful that we have had the experience of her artistry for two seasons; for reasons, however, which the musical press and public can well understand, the Metropolitan is nevertheless grateful that the association is ended."

And, regrettably, so it was, except two "Toscas" a few years later, when her voice was in decline.

But on the whole the company's artistic leadership has been quite distinguished.

That was not the chief priority of its founders, who were the "robber barons"--the post-Civil War New York nouveau riche. Their purpose was clear and simple: conspicuous consumption. The Old Order, in the opera company farther downtown at the Academy of Music, would not surrender the best boxes, and the Vanderbilts, Morgans and Astors needed a place to display their new-found wealth. So they built the Metropolitan at Broadway and 39th. For social safety's sake, there were an incredible 122 boxes for the bejeweled rich. Initial standards might have been higher. Even though a French opera, Gounod's "Faust," opened the house, everything was sung in Italian during the early years. But soon the policy was to outbid the European companies for the leading artists and to get everything that money could buy. (Curiously, the Met now pays considerably lower fees than some other companies, but it is so powerful an institution that performers do not snub it.)

By the time general manager Giulio Gatti-Casazza arrived in 1908 to begin a regime that would continue until 1935, the Met's standards ranked with any in the world, and, with some fluctuations, they would remain at that level. Canadian tenor Edward Johnson succeeded him, staying until 1950. Then came Bing. His 22 years were turbulent but his accomplishments were enormous. He finally gave Mozart his proper place in the repertory, and introduced much Verdi that was then little performed. Bing's grandest accomplishment was the development and construction of the new Metropolitan Opera at Lincoln Center. It opened in 1966 and is probably the most elaborately equipped theater in the world. It is the one building at Lincoln Center so successful acoustically that it has needed no renovations.

One indication of its place in American life came during the fiscal crisis it faced a few years ago. The late Nancy Hanks, then the chairman of the National Endowment for the Arts, was asked what the federal government would do if it appeared the Met was going under. First, she replied, "Well, it is not our policy to salvage failing institutions." Then she paused, and finally added, "But, you know, if we ever make an exception to that rule, it would be for the Met."