Tonight's broadcast of "In Performance at the White House" opens the national celebration of the Metropolitan Opera's centennial.

It was taped in the East Room last Sunday before an invited audience and is hosted by Leontyne Price, whom the company's newly designated artistic director James Levine--who does the piano accompaniments--describes without fear of exaggeration as "one of the greatest singers of all time."

Price, who will preside over all this year's White House concerts, has also been one of the major figures of the first century of our greatest opera company.

This broadcast, which precedes Wagner's "Die Walku re" at 8 on Channel 26 and will also be simulcast on WETA-FM (90.9), spotlights six young singers from the Met's Young Artists Development Program who are described by the typically charismatic and bejeweled Price as "some of the stars of the Met's second century."

At least two of them, dramatic tenor Timothy Jenkins and soprano Marvis Martin, are close to being stars already. Jenkins was the splendid Macduff here last spring in the Met's "Macbeth" and Martin sang ethereally last fall at St. Matthew's Cathedral in the world premiere of Gian Carlo Menotti's "Cantata" on a text by St. Teresa.

In the White House concert Jenkins sings a noble, sonorous performance of Samson's lament from Saint-Sae ns' "Samson et Dalila." He is hailed by Levine, who cast him successully in the title role of "Parsifal" last season, as "the finest dramatic tenor on the scene."

Likewise, Martin is genuinely radiant in "Depuis le jour" from Charpentier"s "Louise." Her ravishing soft high notes have been reminiscent of Price herself in the past, and she must have been thrilled to hear the ultimate judge herself tell the White House that Martin's voice is "something special" and that she has a "most marvelous future."

There is at least one other young singer in the same category of vocal splendor as Jenkins and Martin. Her name is Karen Bureau and she is one of those most valued of operatic species, the Wagnerian soprano. She sang a blazing "Dich teu're halle" from Wagner's "Tannhau ser," marred only by a slight bit of strain on the high note near the end--but otherwise an assured, powerful young voice.

Also: baritone Terry Cook sings Figaro's lament at the folly of women from Mozart's "The Marriage of Figaro" with force though not the ultimate in vocal amplitude; mezzo Gail Dubinbaum does a florid version of the big aria from Rossini's "La Cenerentola," and baritone Brian Schenyder performs Valentine's aria from Gounod's "Faust" with warmth and lyricism.

If these singers are typical of what's coming, the Met will still be in fine shape when all the dozens of virtuosos who will star in the epic Oct. 22 centennial benefit are retired.

Nancy Reagan introduces the concert with a tribute to the Met. "The Met is opera to Americans," she says. "A century is quite a span, and what a century it has been!"

At the end, the president adds, "Luckily the Met has been there all these years." Then he adds in jest that what he knows now about opera is different from his first knowledge of it, "when I was first exposed to opera in the movies, by singers like Lily Pons and Rise Stevens. For years I thought you couldn't give 'Il Trovatore' without the Marx Brothers."

Price also sings. At the beginning she is in fine form with that great signature aria from Puccini's "La Rondine" and at the end she is joined by five of the young singers in the epilogue from Mozart's "Don Giovanni."