Leave it to Robert Bass and the Collegiate Chorale to pounce on an operatic rarity. Tonight in Avery Fisher Hall they will give the American premiere of a Handel opera, “Giove in Argo,” a work that only recently was deemed unperformable because significant portions were thought lost.

” ‘Giove in Argo’ (Jupiter in Argos) has 10 choruses,” Mr. Bass explained in an interview in his apartment on Riverside Drive. “It was written when opera was losing popularity in London and Handel had already started writing oratorios. It was part of the same season as [the oratorios] ‘Israel in Egypt’ and ‘Saul.'”

For the season in question, 1738-39, it was initially unclear whether Handel would muster sufficient solo singers to produce opera. But he did have a chorus, and when enough opera singers eventually materialized, he chose to create a hybrid work that involved both types of performing forces.

“Giove in Argo” is pasticcio — a work assembled from pre-existing music — but, with the exception of two arias by the Italian composer Francesco Araia (only one of which will be performed tonight), the musical numbers are by Handel, who fit them to a bucolic libretto that deals with the chief god Jupiter’s insatiable appetite for mortal women. Handel lovers will recognize several choice arias extracted from other operas, but “Giove” also has some significant new music by Handel. The opera will be performed in an edition by the Handel scholar John H. Roberts, who tracked down missing pieces of the score and composed secco recitatives to supplant Handel’s lost ones from Acts 2 and 3. “Giove” was first given in the Roberts edition at the Handel Festival in Göttingen, Germany, last May.

Not surprisingly, as Mr. Bass noted, any opera taken up by the chorale must have a significant choral component. Yes, the Collegiate Chorale is a choral group, but opera gained a place on its agenda not long after Mr. Bass assumed leadership in 1980, and New York has been the richer for it. Most Handel operas, however, don’t call for much of a chorus. “They usually have just a brief chorus at the end,” Mr. Bass said, and it can usually be performed by the soloists.

Although tonight’s performance represents the first time the Chorale has reached back to the Baroque era for an opera, performances of major choral works by Bach and Handel are bread-and-butter operations for the ensemble, which was founded by the legendary Robert Shaw in 1941 and led by him until 1954. From that point, it was headed by such illustrious choral maestros as Margaret Hillis and Richard Westenberg before Mr. Bass took over.

Mr. Bass acknowledges that not all choristers, at least initially, were happy with his emphasis on opera. But he insists that it gives the singers new insights into choral music. “When I took over, the sacred music tradition was strong, but opera and American musicals were ignored. Performing works from these other traditions helps deepen our understanding of the relationship between words and music. The palette of color changes and expands when the singers experience the full spectrum of the vocal arts.”

Mr. Bass is a native New Yorker and a 1977 graduate of the Mannes College of Music, where he majored in conducting. Three years later he became music director of the chorale, which has remained his base of operations ever since, supplemented by a moderate amount of guest conducting. The chorale’s first opera under Bass was the American premiere in 1987 of Dvorák’s “Dmitri,” and just last month the group offered the New York premiere of “A White House Cantata,” a concert adaptation of the ill-fated Leonard Bernstein-Alan Jay Lerner musical from 1976, “1600 Pennsylvania Avenue.”

But everything was imperiled when Mr. Bass was diagnosed with amyloidosis, a disease that, he explained, settles in an organ and debilitates it. Mr. Bass had a heart transplant in March 2007; the disease also required chemotherapy and stem cell transplants. “Only eight people out of a million get it, and a lot of doctors don’t even know about it. I want people to be aware it, which is why I talk so openly.”

The chorale has been a mainstay of the Verbier Festival in Switzerland, where last summer Mr. Bass resumed conducting after his surgery. This summer, he will take the chorale to Israel, where it will perform with the Israel Philharmonic under Zubin Mehta. Another full roster of New York concerts, including Verdi’s “Luisa Miller” with the Canadian soprano Adrianne Pieczonka, is in the offing for next season. “Music is a completely positive force for me,” Mr. Bass said, “as long as I have enough energy.”