IT IS ACT IV of Shakespeare's "A Midsummer Night's Dream." And it is also the end of the play's central event, that midsummer night snooze in which the prospective marital affiliations of three couples of the Athenian social scene are utterly entangled without the principals' knowledge. Oberon, the god of the fairies and the guiding influence of this comic maze, joins his queen, Titania, and declares:

"Sound, music. Come my Queen, take hands with me,

"And rock the ground whereon these sleepers be.

"Now thou and I are new in amity,

"And will to-morrow midnight, solemnly,

"Dance in Duke Theseus' house triumphantly,

"And bless it to all fair prosperity,

"There shall the pairs of faithful lovers be

"Wedded, with Theseus, all in jollity."

Then Theseus, Duke of Athens and the play's ranking secular figure, almost immediately joins the chorus of commands for music, proposing that "my love shall hear the music of my hounds." This robust message may lose a little when out of context, because "the music of my hounds" that is coming next, at least when the play is performed with Mendelssohn's mesmerizing incidental music, is the Wedding March. That's the one that comes at the end of most weddings -- because its exalted jubilation gives hope that the marriage will at least start off with the right key signature.

The main obstacle that prevents Shakespeare's masterpiece from routinely being played in tandem with Mendelssohn's masterpiece is that theater companies can't afford a full orchestra and chorus and orchestras can't afford a cast of players. The National Symphony is no exception. But this week they will be doing their best, as Mstislav Rostropovich, joined by the women of the Oratorio Society, conducts highlights of the 13 pieces Mendelssohn wrote for performances of the play. They perform tonight, tomorrow night, Friday and Sunday at the Kennedy Center Concert Hall.

Such is the intensity of Shakespeare's lyricism in "A Midsummer Night's Dream" that some might argue that the words are music enough. But the unabashedly romantic atmosphere of the play, with its frequent sharp switches of mood, is aided by musical transitions (many of Mendelssohn's finest movements join acts and scenes).

By the time he had finished this music, Mendelssohn had created one of the two supreme large-scale musical works inspired by Shakespearean comedy. The other, of course, is Verdi's final opera, "Falstaff."

Mendelssohn's musical cornerstone is the brilliant overture, one of the most famous and most utterly original ever written. In four slow, rising, glowing soft wind chords that open the overture, the composer sums up the nobility, love and ephemeral nature of Oberon and Titania, the king and queen of the fairies. Then in the first main theme he evokes, with busy, rustling figures in the high strings, there are the elves, spirits and fairies who do the benign bidding of Oberon. And so on . . .

What is most extraordinary about this overture, though, is the fact that Mendelssohn wrote it at 17. It is a work of such maturity and sophistication that Mendelssohn was plagued for the remainder of his 41 years with complaints that he was failing to match it. No other composer comes even close to having achieved such a teen-age feat, not even Mozart.

The rest of the "Midsummer Night's Dream" music was composed 17 years after the overture, on commission of Friedrich Wilhelm IV of Prussia. Some of the material from the overture is cast in new forms and the rest is new. This was one of those occasions in which the fire that first stirred Mendelssohn flamed fully again.

Music lovers who know the music better than the play would do themselves favors to read it before hearing this week's concerts. It's the only way to grasp the magnitude of the Shakespeare-Mendelssohn partnership. For instance, there's the moonlight and magic of the nocturne that connects the woods scenes of Acts III and IV.

And, unforgettably, there's the soft, delicate, wry and fiendishly difficult scherzo, which is a character study of Puck, after whom comes the work "puckish." It is to him that falls the play's most famous line: "Lord, what fools these mortals be!" That is what Shakespeare's play is about, and that is also what Mendelssohn's music is about.