Wolf Trap opens Monday night with the annual visit of the Metropolitan Opera Company to begin a summer that will bring some unusual concerts to the big shed in Fairfax County.

Strong casting and several of the Met's leading conductors give the opening week of opera no little promise. For the first night, Verdi's "Trovatore" is headed by Renata Scotto, Elena Obratsova, Carlo Bergonzi and Cornell MacNeil. If you agree with the school of thought that puts Bergonzi at the very top of today's Italian-wing tenors, you are immediately impressed with a group of singers that also includes Obratsova, the Soviet mezzo who won herself such immense ovations at the Kennedy Center last year with the Bolshoi Opera.

Her presence on Monday night rules out her singing the role of Bertha in Tuesday's "Le Prophete," which is all to the good, because Rita Shane, who will sing it, is a coloratura soprano who finds no difficulty in handling the high tessitura that pushes Scotto much too hard. "Prophete" is principally, however, the great vehicle for Marilyn Horne, who makes the once-famous role of Fides a phenomenal display of old-fashioned greatness. James McCracken sings the title role, making strong and convincing use of falsetto for the high phrases.

Not to be overlooked in the opera is the recurring chorale," Ad nos ad salutarem undam," sung repeatedly be three of the scruffiest looking characters you will see in any opera. The melody of this trio, decent enough as Meyerbeer sets it out, is the theme on which Franz Liszt later constructed one of the most impressive compositions of his entire career, a Fantasy and Fugue that is played on either piano or organ.

The week continues with "Tosca," "Lohengrin," "Boheme," "The Magic Flute," and a repeat of "Trovatore." The "Flute" on Saturday afternoon is strongly cast, with Washington soprano Carmen Balthrop, who won the Met's top audition prize last year, as Pamina. She will be joined by Shane, Donald Gramm and Jerome Hines, with Frank Conlon conducting.

The Metropolitan Opera is, however, only the first of numerous operatic attractions coming to Wolf Trap this year.

In what is by far the largest amount of opera to reach the park, the summer will include performances of the great Busoni masterpiece "Doktor Faust," with Frank Corsaro in charge of the staging; a Metropolitan production of "Hansel and Gretel," staged by Nathaniel Merrill, and the Santa Fe production of Cavalli's "L'Egisto," to be directed by John Cox from Glyndebourne. Balthrop will return for that opera.

Even those do not complete the Wolf Trap opera list, since Gian Carlo Menotti's double bill of "The Medium" and "The Telephone" will be given by the Wolf Trap Company in the Madeira School.

There is special interest in the coming of Raymond Leppard to Wolf Trap. He is the distinguished British conductor - among other things a specialist in baroque performances - who has contributed notably to the recent liveliness in concerts and recordings of music from Monteverdi to Handel and Bach. On June 28 and 30 Leppard will conduct concerts that will present all six of the Brandenburg Concertos together with cantata excerpts, while on July 3 he will lead a Handel concert.

Julius Rudel will conduct the National Symphony on July 7 and 8, with Martina Arroyo as soloist the second night in arias by Verdi. The concert on July 7 will include the Mahler Fourth Symphony with Janice Hall as soloist. Rudel will return on Aug. 14 to lead a Mozart-Mahler evening in which he will enjoy the solo arts of mezzo Frederica von Stade and baritone Richard Stilwell. Pinchas Zukerman, who began his brilliant career as a violinist of special gifts and then demonstrated that he could play the viola at least as well, will add a third dimension to his already impressive stature on June 29 when he appears with the National Symphony as both violinist and conductor in an all-Mendelssohn program.

One of Wolf Trap's most unusual innovations will arrive July 31 when Paul Traver conducts Handel's "Messiah" in what he is preparing as a performance much like the original, which Handel first conducted in Dublin in 1743.

A real departure from its standard format will be the all-Chopin concert by Earl Wild on Aug. 4. With the greatest respect to Wild's Chopin, which is, like anything he plays, superb, it would seem that in Wolf Trap's larger spaces, Wild would be most appreciated in some of the spectacular 19th-century warhorses in which he excels.

Jean-Pierre Rampal's visit to Wolf Trap last year was one of its special triumphs. He will be back Aug. 21 with harpsichord and piano support, which will let him range through his entire repertoire, from baroque to contemporary.

And for a seasonal finale, preceded by all kinds of dance and popular, folk and jazz nights I have not mentioned, Wolf Trap will bring down the New York Philharmonic under the baton of - guess - Sarah Caldwell, with Andre Watts as soloist in the Tchaikovsky Concerto. All in all, Christopher Hunt, who is new to Wolf Trap this year, has put together a highly promising summer.