WITH MSTISLAV Rostropovich at the National Symphony, there's an undeniable air of excitement and anticipation among Washington's concertgoers. It will be a pity, however, if the obvious growth potential of the new era is blunted at the outset by outlandish expectations or parochial hero-worship.

It is one thing to read in Time and Newsweek encomiums implying that the symphonic millenium is already at hand, or to gloat over the knowledge that the mere name of Rostropovich may finally make the rest of the world take notice of the NSO.

It is quite another thing to hear recent NSO concerts under Rostropovich, as he begins his first regular season as the orchestra's leader, and to listen to them in the context of the broad symphonic universe beyond Washington city limits. A full-page promotional advertisement for the Time cover-story on Rostropovich several weeks ago would have had us believe that "The NSO today seems to be a born-again orchestra which has found a god." Ears insulated from hype, however, tell us that something a bit more earth-bound is taking place.

Within less than two weeks, and given the accidents of concert scheduling, I was able to hear two NSO programs led by Rostropovich, Sergiu Comissiona conducting the Baltimore Symphony, Eugene Ormandy directing the Philadelphia Orchestra and Seiji Ozawa at the helm of the Boston Symphony. That kind of proximity affords a rare and enlightening perspective.

The orchestra are hardly of equal caliber, of course. The Boston and Philadelphia ensembles, clearly among the world's most distinguished, are secure in their technical mastery and refinement. The Baltimore Symphony and the NSO stand on a rung below the top, still raw edged,, but vibrant with the will to conquer new ground.

The Boston is the mellowest of the four, closes in sonority to the "continental" ideal set by the great European orchestras. The Philadelphia, sleek and lustrous and somehow more "American," is a marvel of svelte virtuosity. The lean, brilliant Baltimore Symphony approaches the model of the old Cleveland Orchestra under Szell in its taut and incisive ensemble. The NSO is more akin to the New York Philharmonic or the Chicago Symphony, emphasizing like them such qualities as gutsiness and drive.

The conductors are no less contrasting, in aage, experience, temperament and style. Ormandy, like Rostropovich, has a special way with Russian music, but for totally different reasons. Unlike the latter, he's notably unspectacular on the podium, and the epitome of tasteful moderation musically. The youthful Ozawa is technically adept and cuts an alluring figure as a conductor, but he's rather mundane as an interpreter. Commissiona, the least well known of the group, is slightly younger than Rostropovich and abundantly gifted. He sometimes leans toward overly brisk tempos, but he combines unusual sweep and dynamism with an exacting sense of rhythm and form.

Rostropovich enters this company as a relative newcomer. A masterful list, he has yet to confront the major portion of the standard symphonic repertoire from the podium.

He gives commands with great physical address and unmistakable authority. He can be a ball of fire to watch, in the heat of performance, and he plainly ignites the enthusiasm of the players. At the same time, he frequently lets his passion get the better of the musical architecture, and he's better at summoning instant impact than at sustaining lengthy development in his rush to deliver the emotional goods, he's apt to release the reins much too quickly, as he did, for instance, in the celebrated "Battle on the Ice" movement from Prokofiev's "Alexander Nevsky," which thereby lost most of its chilling intensity.

Listening to four orchestras in juxta-position, however, also reveals how diverse and subtle are the components upon which comparisons must inevitably rest. The four ensembles and their directors are vastly different, but the differences reside in minute degrees of tempo fluctuation of linear clarity, sectional balance, agogics, tonal felicity, articulation and that elusive ingredient known as style, among others.

Nor is there any denying the importance of external factors - acoustics, for instance, or humidity. Baltimore's Lyric Theater is nowhere near as conducive to the tonal enhancement of that city's orchestra as the Kennedy Center Concert Hall, for all its unneven qualities, was to the sound of the other three ensembles.

Musical affinities count for much. Ozawa, for example, might have seemed much less stuffy with Berlioz than he did with the Brahms Requlem, which he gave a rather sententious reading. Ormandy's Mendelssohn (the "Scotch" Symphony), for all its momentum, had a schoolmasterish tinge about it that never enters his Rachmaninoff performances.

Commissiona was dealing from strength in Ravel's "Daphnis and Chloe" Suite No. 2, which he swept to an ecstatic culmination in a gradual, smooth climb. So was Rostrpovich in the brooding rhetoric of Tchaikovsky's "Manfred" Symphony, which made his Brahms D Major Symphony, on the same program, and his Schumann C Major, a week later, sound stodgy and stylistically unconvincing by contrast.

Each musicians has his weaknesses as well as strong points, and these observations suggest again how premature it is, after less than a month, to draw conclusions about Rostropovich's eventual stature as a symphonic interpreter, or about his ultimate effect on the NSO.

When Newseek informs us that "For the National Symphony, Rostropovich has the look of a messiah," and that after a single concert he has "demonstrated beyond a doubt that his Olympian virtuosity is every bit as great on the podium as it is on the cello," it is hard to know what to make of it, besides an overweening urge to sell magazines.

Rostropovich has made some fine music with the NSO so far, though he has yet to get the orchestra sounding better than it did - under some previous conductors, including his predecessor, Antal Dorati, and such guests as Julius Rudel. He has proven box office appeal, unquestionable personal charisma, and the insight of an exceptionally distinguish career as a performing artist. It is entirely possible that he may some day work symphonic wonders with the NSO. But it's hard not to feel that this would have a much better chance of occurring if we were to let him make the attempt in an atmosphere free - before the fact, anyway - of too much hysterical adulation.