With less than five hours' notice, American soprano Olivia Stapp dropped out of her afternoon rehearsals as Lady Macbeth at the Metropolitan Opera in New York, hopped on the 5 o'clock shuttle and subbed for Shirley Verrett as Tosca last night with the Washington Opera.

She was just sensational.

It was a grand moment for the Washington Opera to be able to adjust to adversity with such assurance. The only problem it leaves for executive director Martin Feinstein is that Stapp's Tosca is considerably more powerful than Verrett's polished, but more staid, version.

By the next performance of Puccini's "Tosca," which will be Saturday night, Verrett has said that she will be back. She had gone to New York for treatment of a drug reaction to an allergy treatment. Meanwhile, Stapp is supposed to return to New York for the Met's soon-to-be-introduced production of Verdi's "Macbeth," in which she is sharing the part with Renata Scotto.

Last night was a last-minute-substitute story to compare with the best of them. Stapp, who gained much praise for her Lady Macbeth here last year, arrived at National Airport after many in the audience had arrived at the Kennedy Center.

The only way she could rehearse was between acts, and yet -- remarkably -- her Tosca was quite different in dramatic specifics from Verrett's. Fortunately, she had sung the part with tenor Carlo Bergonzi once before -- in Hamilton, Ontario.

There were practical matters, like costumes. Verrett is a larger person, so Stapp could not use her costumes, except for the capes. Her dress in the first act was the one designed for soprano Teresa Zylis-Gara for Verdi's "A Masked Ball" several years ago. Subsequent costumes were sewn in on the spot.

The opera started 30 minutes late and the breaks were unusually long, due to the rehearsals. Stapp had time to confer with conductor John Mauceri and with Bergonzi and baritone Charles Long only about the barest details of the action.

Just for safety's sake there was a beneath-the-stage prompter, and he must have been one of the highest paid ones in history. Because the company doesn't normally have one, Feinstein prevailed on Cal Stewart Kellogg, the conductor of the new production of Verdi's "Falsaff," to do the underground honors.

It sounded like the scenario for a discreetly-to-be-tolerated mess. It was, in fact, one of those rare performances that was extraordinarily electric. It made the carefully rehearsed version that opened Friday seem bland, solid as it was.

As any great "Tosca" should be, the title character was the center of the action. Stapp has grasped that multiplicity of mood that makes Tosca one of opera's most compelling soprano parts. The portrayal was always in sharp focus, as she switched from confrontation to confrontation.

The memorable dramatic moments are too numerous to itemize. Consider just the end of the second act, after she grabs the pen knife from Baron Scarpia's desk and murders him before he can rape her. Her fury abates, and only then does she look down at her hand. She seems almost shocked at the sight of the knife. She is so revolted that she compulsively tosses it toward the wall. This was one of many telling dramatic details that gave this performance an intensity that apparently is beyond the capable Verrett.

There was also the subtle and passionate shading of Stapp's voice. One would swear that she has been listening carefully to one of the Callas recordings. And what singer would take on the part without taking on that transcendent interpretation?

Bergonzi was also in better form than on opening night, when he was suffering from a cold. And Long's severe Scarpia seemed to gain from Stapp's passion.

There was one thing that differed very little from Friday night's performance. It was Mauceri's conducting, which was really first rate.

One could go on and on, but suffice to say that the Washington Opera and director Feinstein were looking very, very good last night.