It was no soprano who let out the high-pitched “Johohohoe!” on Thursday afternoon as the Metropolitan Opera Orchestra rehearsed Wagner’s “Der Fliegende Holländer.” It was the conductor Yannick Nézet-Séguin, leading his first rehearsal since being tapped to become the Met’s next music director.

This was an orchestra-only rehearsal, so Mr. Nézet-Séguin, 42, sang out odd bits of the text — sometimes in a tenor voice and sometimes in a decent falsetto. He paused occasionally to give instructions — “more bell-like,” “now rougher” — offering a glimpse of how he is shaping “Holländer” (“The Flying Dutchman”), which opens on April 25.

The baton does not pass often at the Metropolitan Opera, which is facing challenging times. Mr. Nézet-Séguin’s predecessor, James Levine, held the post (and near-absolute power) for four decades and only reluctantly stepped down to an emeritus position last year, after a long series of health problems. Attendance is down, and perilous negotiations with the company’s labor unions are on the horizon. The busy Mr. Nézet-Séguin, already the music director of the Philadelphia Orchestra and booked up years in advance, will not officially start at the Met until 2020.

So he gave the orchestra players a short, resolute speech in a rehearsal room three floors beneath the Met’s stage. “I said yes to this incredible challenge for the reason that I am passionate about the art form we’re doing,” he said before the downbeat. “I want even more people to love it.”