The idea for a magnificent musical score to go with the movie, "Alexander Nevsky," did not originate with its composer, Sergei Prokofiev. It was a nonmusician, the giant of Russian directors, Sergei Eisenstein, who saw the potential for this work, with which Mstislav Rostropovich will end the National Symphony's season in four performances this week, the first of which is tonight.

When "Nevsky" went into production in the late '30s, Prokofiev had just returned to Russia from a trip to the United States that included Hollywood, where he saw that musicians of modest talent who applied their musical cosmetics to American films were lavished with instrumental resources that rivaled the ones then available to Toscanini on the other side of the continent.

He saw a potential in film that he had not previously realized, but it remained for the director of "Potemkin" to perceive that in this intensely anti-German movie (done one year before the Nazi-Soviet pact) the music could transcend a "background" role and become primary in an operatic sense.

A little later, Eisenstein wrote, " Prokofiev is a man of the screen in that special sense which makes it possible for the screen to reveal not only the appearances and subjects of our objects, but also, and particularly, their special inner structure . . . Having grasped this structural secret of all phenomena, he clothes it in the tonal camera angles of instrumentation, compelling it to gleam in shifts of timbre, and forces the whole inflexible structure to blossom into the emotional fullness of orchestration."

A little wordy, maybe. For somebody who cares about music it can be explained more simply. From the seven-section "cantata" that Prokofiev drew out of the film, which will be played tonight, we have this: The most stirring Russian nationalistic choral writing since "Boris" (which means the finest such writing except for "Boris"), some descriptive orchestral music of remarkable intensity and an aria--in which a young bride searches through the battle dead seeking the body of her young mate--that is one of the great songs ever written for a mezzo.

The subject of all this is a figure as noble to Muskovites as Patrick Henry is to Americans. As R. D. Charques explains in "A Short History of Russia," " Nevsky is one of the holy names of medieval Russia--he was indeed canonized by the church--and before and during the Second World War was the most heroic of ancient names in the Soviet hagiography. Prince of Novgorad, it was he who, after defeating the Swedes in 1240 on the Neva (hence his title), two years later won the legendary and decisive victory over the German knights on the ice of Lake Peipus."

It is the 12-minute drama of that battle, and its extraordinary music, that is the centerpiece of "Alexander Nevsky." The array of, literally, icy-sounding grace notes, glissandos and percussive effects is unprecedented in music.

Also unprecedented is the notion that a composer should write an aria for a film, such as the young bride's after the battle on the lake, dispensing with the function of film music purely for mood purposes and making the song the focus of attention. Prokofiev never wrote anything finer in any of his splendid operas, and the text (the source of which I have not been able to uncover) is poignant indeed: "I shall go across the snow-clad field; I shall fly above the field of death; I shall search for valiant warriors; My betrothed, my stalwart youths . . ." Lili Chookasian will sing it tonight; she has been doing it memorably for years.

The sonic splendors and lyric beauty of "Alexander Nevsky" have had a natural appeal for conductors since it first appeared (Eugene Ormandy also is concluding the Philadelphia Orchestra season with the work).

Though Rostropovich has yet to record it, many others have--and most of them very well. A brief survey of some of these records shows at least four of distinction--the St. Louis Symphony under Leonard Slatkin, the London Symphony under Andre' Previn, the Chicago Symphony under Fritz Reiner and the London Symphony under Claudio Abbado.

Two of them, the Reiner and the Abbado, are brilliant. Choosing between the two is not hard; either is so fine that you can't lose. I would lean toward the Abbado. It is in Russian (the Reiner is in English) and his mezzo in the bride's song is the superb Russian mezzo Elena Obraztsova, singing with subtle shading that will take your breath away. But I have no plans to discard the marvelous Reiner.

A final downbeat note: If it is almost uncontestable that no one had written a film score of this stature before Prokofiev, it is equally true that no one has done so since.