Wolf Trap presented a concert version of Franz Lehar's durable operetta, "The Merry Widow," Saturday night. In spite of the generally undesirable mugginess of the weather, the hillside was full and almost all the seats in the Filene Center were occupied.

Lehar's 1905 triumph is a formula piece in the tradition of Johann Strauss and Franz von Suppe, about as predictable as any Perry Mason novel. The formula is an effective one, made up of good tunes, accessible harmonies and piquant rhythms, surrounding a mindless plot concerning intrigue and romance among the high-born in some fictitious Mittel-Europa country on the order of Graustark or Ruritania. The recipe works, partly because many more than two hearts beat in three-quarter time.

Of course, a concert version is not entirely satisfactory. Absent are the costumes, the dance, the broad-brush acting, the swirling movement of many people and the lighting effects, all of which combine to add a visual dimension to the production, taking the mind off the music, which is pretty poor stuff, even if the orchestration is superb. Lehar was well enough off to farm out the routine work of scoring to a much better, if less wealthy composer, Arnold Schoenberg.

At Wolf Trap, the soloists were competent, the orchestra adequate and the chorus surprisingly good.

Among the leads, first honors go to John Reardon, who sang well and who made the most of the vestigial opportunities for acting possible in a concert version.

The justly famous Anna Moffo was not very impressive, except in "Vilia," one of the great show tunes, which she sang with conviction, with vocal mastery and with a sure sence of style.

David Kuebler as Camille de Jolidon displayed a pleasant tenor voice with a good high range. Faith Esham, substituting for ailing Karen Yarmat as Baroness Popoff, has a nice voice and acted with courage, if with a certain awkwardness. I predict a good future for her.

Veteran Franz Allers conducted the performance with thorough knowledge of the music, and with a real professional knack for glossing over the routine parts and making the most out of the big moments. His beat resembles an industrious cobbler half-soling a pair of bluchers, but he kept things well in hand. If one must make a choice precision is better than grace. Allers also narrated, as it is called.

The show was sung in English, to make everything clear to the audience. Unfortunately, the diction Saturday night, not helped in any way by distortion in the amplication, made things so obscure that cast and chorus might as well have been singing in Urdu or Estonian. Small loss. The libretto gets low marks as literature, and Allers kept us well aware of the intricacies of the plot.