Leonie Rysanek's magical evening at the Met

In opera, 25th-anniversary concerts often conjure up the celebration of an artist past his or her prime, offering the best of what is left to an adoring public. Rarely does such an event - with the incredible pressures it puts on the artist to make the best of an emotionally charged evening - become anything more than a sentimental journey.

That said, Leonie Rysanek's gala celebration of 25 years with the Metropolitan Opera was one of the outstanding musical events of my concertgoing life - one of those magical evenings where everything went right. The Viennese diva was in incomparable voice. And her famous histrionic powers were provocatively channeled to bring two radically differing Wagnerian characters to life in one evening.

It seemed like a grueling idea to have the soprano sing Kundry from the second act of ''Parsifal'' and then Sieglinde from the first act of ''Die Walkure.'' The two characters are utterly different - the former a complex siren/seductress, the other a passionate, love-filled young woman. Kundry, which has stretches of high singing, essentially lies low for a soprano. Sieglinde, however, flatters all the glories of Miss Rysanek's effulgent upper register. Her Sieglinde has been the standard-setter for over 30 years; she added Kundry to her repertoire only in 1976.

Thus it was the Kundry that was the novelty to the audience at the Met (where she will sing the entire role next season). To do justice to Wagner's incredible creation, a singer must be able to communicate the mythic as well as the human, the cruel as well as the loving, the proud as well as the fearful. In concert dress, standing in front of the Met orchestra, Miss Rysanek took on all the aspects of this creature, from the opening agitated pages of the act to the cataclysmic end. Her histrionic powers - already the stuff of legends - took on an added dimension of power and communication. The singing was unstintingly magnificent - rounded in the upper reaches, powerful and penetrating in the lower register, and throughout the entire range a marvel of shadings, of tonal variations, of coloristic nuance and textual clarity.

After intermission, Metropolitan Opera Association president Frank Taplin presented the singer with a silver tray - and with one of the warmest (and most accurate) tribute speeches to be heard at such an event. She managed to get a few words out, paraphrasing Hans Sachs from ''Die Meistersinger'': ''For you it is easy, for me it is very difficult accepting all this love. I try to give my best, and if it is not my best, at least it's from my heart.''

And there followed a performance of Sieglinde that had the vocal freshness of her 1951 recording (with Wilhelm Furtwangler), along with the incredible insights she has gained from living with this role for almost 33 years. How she found so much voice after having expended so much in the Kundry is but one of the aspects of Miss Rysanek's triumph which ensure this concert a prime spot in the history books of exceptional operatic triumphs. Through it all, she also dared show us her heart, and her vulnerability, in an age that favors stoicism and restraint.

She had wonderful assistance on her very special night. James Levine was the ever-attentive maestro, giving her the musical room to bring these strikingly different women to life. Peter Hoffmann is the best Parsifal and Siegmund around today. Basses Franz Mazura and John Macurdy fulfilled their roles with distinction.

But of course it was Ms. Rysanek's evening, and her fans let her know it - a Met full of cheering opera-lovers, a shower of bouquets and confetti, nearly 20 minutes of awesome, love-suffused ovations. It was the sort of evening that will be talked about for years to come. Happily, it stands only as a landmark: Miss Rysanek is still very much with us, as this evening gloriously attested. 'Arabella'

The Met has revived its visually imposing production of Richard Strauss's ''Arabella'' (new last season), with Kiri Te Kanawa again protraying the heroine.

The singer was in far better voice this year than last, which was something of a surprise, given the way she handled the role of Violetta in Verdi's ''La Traviata'' earlier this season. At that time, as she strained for her top notes , she barely scratched the surface of the role and seemed generally ill at ease. She also insisted on wearing costumes from the old London production of the opera which clashed with the Met's staging.

That tattiness was banished here. She once again looked radiant in the role. The voice was sumptuous in the upper reaches, although rather vague in the lower. But now, more than ever, she treats the evening as a vocalise, never connecting with all the subtle emotions Arabella must communicate if the portrayal is to hold interest.

She was not helped by the Madryka of Norman Mittlemann, who neither looked nor sounded the role of Arabella's country-elegant suitor. Happily, true Strauss magic occurred each time Kathleen Battle was on stage as Zdenka: Vocally and histrionically, this is the finest thing she has done at the Met. Would that David Rendall, as her suitor Matteo, looked the part and sang consistently in tune.

New in the pit was Marek Janowski, who offered a warm, lyrically persuasive account of this difficult score. He is not a flashy maestro, but he is warm, musical, and supportive of the singers, which was not the case with Erich Leinsdorf last season.