MARTIN FEINSTEIN and the Washington Opera have chosen a sleeper -- Pietro Mascagni's "L'Amico Fritz" -- for the Eisenhower Theater's debut this weekend as an opera house.

Mascagni, okay; everybody knows his bloody "Cavalleria Rusticana." But L'Amico who? Honestly, how many people do you know who have been sitting around muttering, "I wish somebody would put on 'L'Amico Fritz' in the Eisenhower.' "

Actually, the chances are that by midnight Saturday, hundreds of Washingtonians will be happy about this daring choice. The Eisenhower holds just over 1,000 seats when its orchestra pit is opened up, and some opening night patrons may be hard-core types who want to see violent passions unleashed and blood shed. But for the unabashed romantic slobs in the audience -- for people who think there should be more operas about nice, ordinary people leading essentially happy lives to the tune of some exquisite music, "L'Amico Fritz" is the answer to a prayer.

It is the story of Fritz Kobus, a wealthy, middle-aged Jewish bachelor and philanthropist in rural Alsace. He hangs out with bachelor friends and vows never to marry -- until Rabbi David tricks him into realizing how much he loves Suzel, a young farmer's daughter. If the summary sounds like a television sitcom, so does the opera, but it is a sitcom with music of limpid, untroubled, idyllic beauty and constantly fresh invention.

There is only one recording of "L'Amico Fritz," but it is excellent. Dating from 1969, it features Luciano Pavarotti and Mirella Freni in their prime, with Gianandrea Gavazzeni conducting the Covent Garden orchestra and chorus. Recently remastered on two compact discs with a handy little libretto, it should satisfy most listeners who want to know this pleasant little opera more intimately. "L'AMICO FRITZ" --

Pavarotti and Covent Garden orchestra and chorus (EMI CDCB 47905). By the Washington Opera at the Kennedy Center Eisenhower Theater Saturday, Tuesday, December 26, 28 and January 1, 5, 7, 9, 11, 15 and 17.