WHEN Leonard Bernstein conducts the National Symphony Orchestra in the second of Brahms' two orchestral serenades in March, he will be, by implication, raising the question of why these works turn up so infrequently in our concert halls. Brahms' four symphonies, his four concertos, his solitary set of orchestral variations and even the German Requiem are all well represented, but audiences seldom get a chance to hear the only other orchestral works by this revered composer.

It certainly isn't because the music is without merit. Brahms is at his heartiest and most ingratiating in his serenades. They are both early works, but masterly nonetheless, filled with endearing tunes and clothed in the handsomest and most imaginative orchestral colors. The one in A major, Op. 16, which Bernstein has programmed with the NSO, is the shorter of the two and is scored for winds and low strings, omitting violins. (It was the model for Dvorak's splendid serenade in D minor, for strings, cello and bass.) The earlier one, in D major, Op. 11, nearly twice as long and scored for full orchestra, has recently appeared in a stunning new recording by Kurt Masur and the Leipzig Gewandhaus Orchestra (Philips 6514.081; cassette 7337.081), and it is one of the year's most treasurable orchestral releases.

From the first prefatory gestures, it is clear that this performance is going to be an outstanding one. Masur's feeling for the work is manifest in ideal tempos, the natural and convincing phrasing throughout the six movements, and the warmhearted enthusiasm with which his fine orchestra and its superb soloists respond. Other conductors have given us attractive versions in the past -- Stokowski, Kerte'sz and Haitink, most notably -- but none has realized this marvelous work so completely. Masur has done nothing finer on records, and the Serenade's continued absence in the concert hall makes this record all the more welcome.

Brahms' only other orchestral compositions, in addition to those already noted, are his own settings of three of his 21 Hungarian Dances, which he composed originally for piano duet. A new recording of the entire set of 21, performed by Franz Liszt Chamber Orchestra of Budapest, with Ja'nos Rolla as both solo violinist and conductor, is one of RCA's Erato imports from France (STU 71411).

While this group has given us some glorious Vivaldi and would seem a likely choice for anything Hungarian, the record is disappointing -- not because of the performance level, but because of the soupy transcription by Frigyes Hidas. His schmalzy and interminable arrangement of No. 4, which opens Side 1, is almost a burlesque of the old gypsy cafe' style, and nothing following is much more persuasive. Pity -- such fine performers, and such fine sound.

Some altogether more palatable Hungarian dances, which have nothing to do with Brahms, are served up irresistibly on another French import, a collection of "Danses anciennes de Hongrie et de Transylvanie" performed by the Clemencic Consort (Harmonia Mundi France HM 1003). With an intriguing assortment of old string, wind and percussion instruments, Rene' Clemencic and his enthusiastic associates give us more than two dozen 17th-century dances whose spirit is closer to the Brahms of the D major Serenade under Masur than to the cloying arrangements of the Hungarian Dances on Erato.

The "Oto dik Tancz," which illustrates this happy sequence, sounds like what might be called old Hungarian Bluegrass, and there is not a single piece on either side without its own little surprises. There are regional and descriptive dances, a few in such courtly forms as the galliard, sarabande and bergamasca, and even some fanfare pieces -- plenty of attractive variety, with the superbly sympathetic performances providing the unifying element of constancy. Superb sound, too, and reasonably informative, if hardly exhaustive, annotation. This one, I think, is a must for almost any kind of collection.