The Metropolitan Opera gave its fans double cause for admiration last Saturday: In the afternoon it presented the Met's first production of Francis Poulenc's great opera, "The Dialogues of the Carmelites," and in the evening a performance of this season's major revival, Meyerbeer's "Le prophete."

The Poulenc opera is 20 years old this year. Its world premiere was at La Scala in Milan in January 1957, followed by its first Paris performances in the summer of that year.

Based on a play by Georges Bernanos, which was in its turn taken from a novel by Gertrude von Ie Fort, the opera is the true story of a community of Carmelite nuns from Compiegne, outside Paris, who were guillotined during the last 10 days of the Reign of Terror in the French Revolution.

But the opera deals with philosophical and spiritual matters that transcend the immediate physical fact of that horror.

Its central figure, Blanche de Ia Force, the daughter of an aristocratic family, enters the Carmelite order a timid but determined young man. She fights the fear of fear itself, and in the opera's final moments, rejoins her sisters at the guillotine, having fled from the convent when the revolutionaries moved against it. The opera probes deeply into the fear of death and the mysteries of those about to die.

It is neither a subject, nor, in its nearly all-women cast, a production that seems likely to succeed in large opera houses. Yet at its conclusion on Saturday afternoon, the applause, which had been largely withheld throughout the long performance, burst out in a roar which has rarely been duplicated. Clearly, since the state was usually filled with various groups of the 16 nuns involved, who often seemed as like each other as their order intended them to be, the tremendous applause was for the total impact of the opera.

This result surprised no one who knew Poulenc's score - his unquestioned masterpiece and one of the great lyric triumphs of the past half century. It is a score of overwhelming beauty, its foundation an orchestral tapestry of exquisite textures and sonorities, over which vocal lines of expressive richness never falter.

The staging by John Dexter is some of the finest ever seen at any opera house, providing remarkable variety in situations of close similarity, and producing an overall sense of deep spiritual meaning which was clearly responsible for the final ovation. Dexter must change only the final moments of the opera where Sister Blanche returns. It is imperative that she and Sister Constance see each other more openly so that the audience can appreciate the full significance of their final meeting. But the Met stage was afire with greatness during this opera.

So was the musical end of things, in the charge of Michel Plasson, making his debut as conductor there. Every phrase of the opera had its proper plasticity, its needed shape.

The strong cast was headed by Shirley Verrett, Regine Crespin, Maria Ewing, Betsy Norden, and Mignon Dunn, with Crespin, Dunn, and young Norden topping the rest in total awareness of their roles.

The evening's "Prophete" was, in comparison, the highest form of operatic camp around these days, made palatable by the magnificent singing of Marilyn Horne as Fides, and superb work from James McCracken in the title role. McCracken handles the long, tough part with immense skill, especially in his knowing use of falsetto and a high, light mixed voice. Horne stopped the show with each of her major scenes. Jerome Hines, celebrating his 30th year with the Met, is a pillar in the role of Zacharias.

Renata Scotto, in the leading soprano part of Berthe, contributed some of the worst screaming sounds heard in many years in any opera house. She should yield the role at once, though to whom would be a tough question.