THE METROPOLITAN Opera returns to Wolf Trap this week its fifth straight year. But, as in each of the past years when the Met has opened the Wolf Trap season, some serious problems seem likely to arise because of weaknesses in conductors and casts.

For instance: Of the four conductors who will be in charge of this week's eight operas, not one conducted any of them during the Met season that ended in New York on April 15.

Of the four, Richard Woitach, who will conduct "Boris Godunov," is the only one whose name appears in any casts of the past season. Michelangelo Veltri, who will conduct "Cavalleria Rusticana" and "I Pagliacci" and Donizetti's "La Favorita," is listed with the year's conductors, but does not appear on any of its cast lists. Gianfranco Masini, the conductor for "Rigoletto" and "Madama Butterfly" is not named among the past season's conductors, nor is Theo Alcantara, who was only named to conduct "Don Giovanni" a few days ago.

The likelihood of achieving anything like fine ensemble work, to say nothing of real cohesion, polish and possibly even greatness is out of the question in these circumstances.

The conductor's stand is not the only place where there will be new faces. In "Don Giovanni," for example, Joan Carden and Elizabeth Harwood are cast as Donna Anna and Donna Elvira. Neither soprano appears on the Met's roste for the past season. And Rockwell Blake, who will sing Don Ottavio, will have made his debut with the Metropolitan on the road two weeks before singing here.

Tenor Gianfranco Cecchele and soprano Mariana Niculescu, who will sing Turiddu and Nedda here are both listed on the company roster for the past season, but neither was named on any casts.

The possibility of discrepancies between what the public hopes to hear and what, in some cases, it is likely to hear, is more than distinct. Three of the opera performances to be given at Wolf Trap were seen and heard from the Met stage in its widely, and deservedly publicized live telecasts of Nov. 7, March 16 and April 5.

The November telecast of "Rigoletto" was conducted by the Metropolitan's music director and leading Verdi conductor, James Levine. His cast was headed by Cornell MacNeil, Placido Domingo and Ileana Cotrubas, Levine will, of course, not be with "his" company at Wolf Trap, and the roles sung by Domingo and Cotrubas will be taken by Neil Shicoff, who was heard on the radio broadcast of the opera, and Judith Blegen, who may turn out to be a better Gilda than Cotrubas.

The television cast of "Don Giovanni" was one of the Met's strongest, with Joan Sutherland as Donna Anna, the new Julia Varady superb as Donna Elvira, and the men headed by James Morris, John Brecknock and Gabriel Bacquier. Morris will sing the Don here, and Donald Gramm makes a better Leporello these days than Bacquier. But Carden and Harwood in place of Sutherland and Varady raise grave questions. Roberta Peters, however, is far more appropriate for the role of Zerlina than the mezzo Huguette Tourangeau, seen on TV.

In the telecast of "Cav," Levine conducted a cast headed by Tatiana Troyanos, Domingo and MacNeil. In "Pag" Levine had Teresa Strates as Nedda, with Domingo and MacNeil again. Here Niculescu will sing the Nedda with James McCracken and Louis Quilico, while Troyanos's Santuzza will be sung opposite Cecchele and Vern Shinall.

The situation surrounding "La Favorita" is the most disagreeable of the entire week. The opera is weak to begin with. In New York it needed every bit of strength of one of the strongest casts to be heard anywhere in the world: Shirley Verrett, Luciano Pavarotti and Sherrill Milnes. Here it will be sung by Bianca Berini, who does not appear on the Met's roster for the past season. She will be joined by John Alexander and Louis Quilico. As Georges Clemenceau once remarked in a somewhat different context, "Quelle chute!"

Boris Godunov" will come off nearer to the New York performance than the week's other operas. Jerome Hines, now in his 33rd season with the Met, will replace Martti Talvela, surrounded by most of those who sang the work a dozen times during the past season, but with Woitach replacing conductor Kazimierz Kord.

The point of all of this is simple: Opera is not only the most expensive of all the musical arts, it is the most difficult in which to achieve and sustain high quality. For the Metropolitan Opera to treat Wolf Trap and this area in the cavalier manner with which it consistently visits Fairfax County is an artistic offense that comes very close to contempt. But the situation will continue until Wolf Trap tells them up in New York, "Put up or shut up." It is long past time they did.