When Leonie Rysanek steps onto the Metropolitan Opera stage next Sunday to celebrate the 25th anniversary of her Met debut, she will be adding another in a seemingly endless series of auspicious landmarks to her remarkable career. The all-Wagner gala will feature Act II of ''Parsifal'' for her blazing Kundry and Act I of ''Die Walkure'' for her legendary Sieglinde.
She debuted Feb. 5, 1959, in a new production of Verdi's ''Macbeth'' at the old Met, a production originally intended for Maria Callas. Her colleagues were the late, great American baritone Leonard Warren, the then-up-and-coming Italian tenor Carlo Bergonzi, and the American bass Jerome Hines. Her triumph was complete. As she said in a recent interview, ''With the Met audience, it was love at first tone! . . . what a cast that was, what stature. The whole production - these were really great performances.''
During her 25 years with the Met, the company has mounted new productions of nine operas for her, four of which were Met premieres. She has sung 216 performances of 18 roles at the Met. ''Here I was not fixed in one certain repertoire. I sang Italian operas, I sang German operas. I could show off all my facets, all the things I can do.''
She loves the Met audience for its loyalty and generosity. But audience generosity toward Miss Rysanek is a worldwide phenomenon. She is the reigning diva of Vienna, where she has sung for 32 years. In Berlin, Munich, Paris, Marseille, San Francisco, Tokyo, everywhere she sings, she brings audiences to peaks of ovational fervor with her performances. And no wonder.
Of her considerable repertoire, there are at least a dozen roles on which Miss Rysanek has put a definitive stamp, among them the Empress in ''Frau Ohne Schatten,'' Leonore in ''Fidelio,'' Senta in ''Der Fliegende Hollander,'' Chrysothemis in ''Elektra,'' Sieglinde in ''Walkure,'' Ariadne, Elsa in ''Lohengrin,'' Elisabeth in ''Tannhauser,'' Lady Macbeth, Desdemona, Tosca. To have seen Rysanek perform any of these roles is to forever remember the blazing impact of her stage presence and the thrilling presence of a uniquely warm, thrusting voice that encompasses a tremendous range of colors, dynamics, and timbres from very top to bottom.
A bit over a year ago, she added a new role to her repertoire - Ortrud in ''Lohengrin.'' Once again, a new standard was set in the role. Miss Rysanek does not so much perform a role as inhabit it. She becomes the role she is playing. Every gesture serves the dramatic impulse of the moment. Every vocal phrase is sculptured to fit that mood. She can imbue the voice with edge when needed, yet overall, one is constantly aware of a uniquely beautiful, communicative instrument unfurling radiantly in a flood of communicative tone.
She is often asked what her secret is. ''There is no secret, really. First you have to have a very robust nature. The material (voice) has to be there (or else) you will not capture people as you do when you have a big voice, and the technique, and the knowledge what to do with it. It's very hard to tell this to people. I don't know: When I go on stage I don't say 'you do this today, and this, you move this, and go there . . .' I don't. It (the role) does that.''
We got to talking about big voices when the discussion came around to her instrument. ''The production of my voice makes you think it is a big voice. I don't think it's such a big voice. My technique is such that my overtones in the voice are so strong, and they make a voice sound bigger . . . not 'uuugggh' - that's wrong, forcing a voice. I always had a good, healthy voice, but compared to Birgit Nilsson . . . that's a big voice.''
Miss Rysanek is beloved by not just audiences but choristers, orchestra players, stagehands. In 1979 she was given the Lotte Lehmann ring by unanimous decision of the performers at the Vienna State Opera. All comment on how youthful her voice sounds. When asked about this she answers: ''Maybe I was wise enough to undersing instead of oversing (throughout my career). I tell this to young singers, and they usually don't listen. They think 'it can't happen to me.'
''I was very lucky. I just caught the end of a great era - great conductors, great stage directors, very careful people saying 'you don't do that, you do this, you sing this, you learn that.' You go in the little houses; you don't start at the Vienna Staatsoper. You have to learn your craft.'' Among the condutors she worked with in the early part of her career were Furtwangler, Erich Kleiber, Knappertsbusch, Bohm, Mitropoulos, Karajan, Kubelik, Reiner, Solti, Serafin. Karajan gave her many new productions when he headed the Vienna Opera. But the major influence and mentor was Karl Bohm. And for Bohm she stepped out of her repertoire to record the title role of ''Elektra,'' which was the last music Bohm conducted before his death. And she has nothing but praise for Mr. Levine, with whom she works regularly at the Met, and at Bayreuth as well.
Next season Miss Rysanek sings Kundry (''Parsifal'') at the Met, and the season after, Ortrud. Next season, she introduces another new role to her repertoire, that of Kostelnicka in Janacek's ''Jenufa,'' a role that might have been written for her. We can also look forward to seeing the movie of ''Elektra'' directed by Gotz Friederich on PBS next season. Miss Rysanek owes her longevity to her caution (and to Bohm's guidance). ''It was very early offered to me to sing Brunnhilde, Isolde, Salome. I said 'Not until I'm 40. (She has never sung Brunnhilde or Isolde, and she performed her first Salome only 12 years ago.) I could easily have sung them, but I'm still around!'' And the Rysanek magic still blazes with the lustre and richness that have been the trademarks of an exceptional career.