"I guess it really did happen that way," said Hugh Wolff, looking and sounding not all all nervous. "The maestro is stricken ill during a performance and an unknown player named Toscanini stands up in the cello section, all ready to conduct 'Aida.'

"I don't expect it to be quite that dramatic."

Wolff's debut last night in the National Symphony's subscription series was not quite as abrupt as Toscanini's in Rio de Janeiro 94 years ago. Wolff, 26, learned on Saturday afternoon that he would have to substitute for Antal Dorati, who is ill, and he was able to conduct three rehearsals ("that's a lot better than coming in after someone else's rehearsal," he says casually).

Traditionally, last-minute substitutions have signalled the start of illustrious conducting careers. It happened not only to Toscanini but to Eugene Ormandy, Leonard Bernstein and a host of others. In Wolff's case, the situation may be slightly less spectacular -- as conductor in residence, he is routinely on call for such occasions. But one would expect the slender, red-haired young man facing his first big concert to be frightened.

Not at all: "One reason I decided to be a conductor rather than a pianist was that I found I was less nervous while conducting," said Wolff. He made his choice between three musical careers five years ago in Paris (where he was born, though he grew up in Bethesda), during a sabbatical year after his graduation from Harvard.

"I was trying to decide between composing, conducting and the piano," he recalls. "I decided against the piano after entering and losing a major competition for which I had spent six months preparing, eight hours a day.

"If wasn't so much that I lost -- everyone loses sometimes -- but I looked at the prospect of spending my life touring, alone eight hours a day with an inanimate object, as opposed to spending eight hours a day with 100 musicians, all of whom have personalities, and I decided to be a conductor."

Wolff was unanimously chosen last summer for his NSO appointment by Mstislav Rostropovich and members of the orchestra after three days of auditions among 14 candidates. By then, he was already a seasoned conductor, having been Leon Fleisher's assistant at the Annapolis Symphony since 1976 and, at the same time, conductor of the Peabody Preparatory Orchestra in Baltimore.

His conducting career began as an undergraduate at Harvard University student opera productions (including Strauss' "Ariadne" and Mozart's "Magic Flute"). In his senior year, before graduating magna cum laude, he was conductor of the Harvard Bach Society Orchestra. He has already conducted, elsewhere, all the music on this week's NSO all-Beethoven program, and he has played the solo in the Fourth Piano Concerto.

When he entered Harvard from Walt Whitman High School, Wolff was "expecting to be a chemist or something," but he already had a strong musical background, having studied piano (with Vida Novick and Leon Fleisher) since his 10th year and composition (with George Crumb and Grace Cushman) while still in high school. His interest in science began to wane, he says, "when I found myself in the same classes with people who were really geniuses at that kind of thing. I felt I was surrounded by people who knew that answers when I did'nt."

But musically, he felt right at home -- in his freshman year, he was taking a third-year course in musical analysis and a graduate-level course in composition with Leon Kirchner.

Earlier this week at the National Symphony, where Wolff has been in residence this season under the Exxon/Arts Endowment program, the musicians seemed happy about his career choice. "I think everyone wants to make this a special night for him," said Linda Harwell, assistant principal bassoonist. "There was a lot of friendly applause in the orchestra when he stepped up to the podium for the first rehearsal, a lot of foot-stamping. I think he's very gifted, and I think that's an opinion shared by most of the orchestra."

Concertmaster Miran Kojian, who is not playing this week because of a minor medical problem with his left hand, described himself as "a great fan" of Wolff. "If he's smart about what to do with his future," he said, "he should develop into a very notable conductor. He's a fine musician, a fast worker and intelligent, very musical and very clear. He knows what he wants, and he doesn't waste time. It's very easy to work for a man like that."

Wolff was scheduled to make his conducting debut in the orchestra's regular subscription series next November, but he has been working with the orchestra all this season under the endowment program, conducting some rehearsals (for example, the Mahler Seventh -- a formidable score -- when Rostropovich was ill) and conducting children's concerts, family concerts and similar educational programs.

"I've been going to all the rehearsals and studying all the scores in case of an emergency like this," said Wolff, "but what I've really been doing is learning. Last year in Baltimore, I attended Sergiu Commissiona's rehearsals and learned a lot -- but this year I'm being paid for it."

When he is not busy with the NSO, Wolff has been taking other assignments occassionally -- most recently, conducting the offstage brass section in The Washington Opera's production of "Tristan und Isolde," in exchange for which Martin Feinstein had given him a ticket to last night's Birgit Nilsson gala.

"It bothers me to miss that," he said, "particularly after I went to so much trouble to get a ticket lined up -- but I'm going to have to cancel my last "Tristan' Friday night. I'll be busy with the NSO. They'll have to get asubstitute for me while I'm substituting for Dorati -- maybe John Mauceri."

Meanwhile preparing for last night's debut, he seemed calm, efficient, totally unworried.

"He's very businesslike at rehearsals," Linda Harwell said after the final rehearsal was ended. "We finished on time today when we had overtime scheduled. When word of that gets around, he should get plenty of invitations to conduct."

At the end of the concert, Wolff was not carried away triumphantly on the shoulders of the audience, the stage was not covered with roses, and there were no reporters of anyone fainting. But approximately half the audience gave him a standing ovation, the orchestra joined in the applause, and it was clear that the evening had been, if not a historic blockbuster, certainly a solid success.