A LONG-HAIRED miniature dachshund named Pooks goes to all the National Symphony Orchestra rehearsals these days. It isn't surprising, since he lives at the Watergate with his friend and master, Mstislav Rostropovich.

Last Monday afternoon Rostropovich conducted his second full rehearsal with the National Symphony Orchestra since assuming the title of the orchestra's music director. He had asked for the double session in one day as a part of his preparedness campaign to get everything in top shape before the opening concert of his first season in the new role.

But before getting into an interview back in his Watergate apartment, Rostropovich excused himself long enough to fix Pooks' dinner, mixing the food and some water and then, very carefully, two drops of medicine.

"It's is for his heart, you know," he explained as he made sure that just two drops got into the food. "His heart is not strong."

Pooks has been one of the lights of Rostropovich's life for several years. If his master says the right words to him in Russian, he will jump up to the keyboard of any nearby Steinway and play. He is equally happy up in the treble region or down at the bass end. When the applause comes, and it always does, he jumps down happily and looks as if theconcert were over.

But as the ovation continues, he begins to look more and more like Isaac Stern or Beverly Sills waiting in the wings, gauging the applause before coming back to center stage. At precisely the crucial moment, just as another Russian phrase wings toward him, he leaps back to the piano bench and gives you the encore you wanted.If Rostropovich ever has any trouble making ends meet either with his cello or waving a baton, he could make a mint on the Johnny Carson show with Pooks.

For all the energy that pours out of him during these critical National Symphony rehearsal days, it is a very relaxed and happy-looking Rostropovich who works carefully through every phrase with his fellow musicians in the orchestra. In speaking of the program he chose for his opening this week, to be played on Tuesday, Wednesday and Thursday nights, Rostropovich said, "You know, it is much harder for the musicians to play a Dvorak symphony beautifully than music by Penderecki or Lutoslawski."

He was making the point that it is more difficult to achieve the kind of lustrous tone he wants in Dvorak than in a recent work by either of Poland's leading composers, despite the great difference in compositional techniques used by the two men. "Dvorak is much more exposed, clearer," he continued, "and everything must go just so."

The cellist, who is now adding the responsibilities and pleasures of the baton to his famous bowing arm, worked precisely and at an almost leisurely pace with the orchestra last Monday when he began work on the Dvorak Seventh Symphony. One facet of his rehearsal technique that aroused some surprise among those who were watching was that he was as at home in discussing technical points with the woodwinds and brass as he was with the string players.

When Rostropovich came on stage, the score of the symphony was under his arm, and he consulted it continually during the rehearsal. When asked about his close attention to the score, in contrast to some conductors who encourage the mistaken idea that it is more blessed to conduct without a score, or at least a sign of greater ability, a better memory or the mark of a superior being. Rostropovich said, "I like to keep eye contact with what the composer wrote. If I do not have the score, I may go too far."

After a week of concerts in which Lorin Maazel conducted Brahms without score and continually departed from the composer's explicit directions, the Rostropovich point of view is highly reassuring to those who agree with him that the composer is the primary source.

Life is never without some disappointments to those who live long enough and make big plans. Rostropovich had hoped to bring his Washington audiences several new works by prominent composers in this, his first season as the National Symphony's musical chief. His closest friends among composers, Dmitri Shostakovich and Benjamin Britten, had both promised to write major new scores for him, but both men died before they could fulfill those promises.

Rostropovich has a letter from Shostakovich in which the great Russian speaks of his plans for a new work. And Britten had written 14 pages of a new composition before his death last year. This week's opening concerts are being dedicated by Rostropovich to Britten's memory.

The death of his English friend and colleague has led directly to Rostropovich's assuming still another new title artistic director of the Aldeburgh Festival in the east coast town that was Britten's home for most of his life. In recent years, both Rostropovich and his wife, the soprano Galina Vishnevskaya, often have taken part in the festivals there. Britten wrote new music for both of them: a song cycle for Vishnevskaya, and for the Rostropovich cello, two suites, a sonata, and a symphony for cello and orchestra.

"When the man who has created such a festival dies." Rostropovich said, in explaining his new role, "they are afraid sometimes that the festival will go down. It was the same after Casals died in Puerto Rico. So they asked me to become artistic director." It is a signal mark of the great esteem as well as the deep friendship that grew up between the Russian cellist and the English composer that the new artistic director at Aldeburgh is indeed Rostropovich.

But those who know how Britten studied Russian so he could write a cycle of songs to Pushkin texts for Vishnevskaya, and who remember also that it is Vishnevskaya who is the soprano soloist in the composer's own recording of his moving War Requiem will not find it surprising that both soprano and cellist-conductor continue to feel closely tied to the Aldeburgh Festival. Among Rostropovich plans for next year's National Symphony programs are performances of the War Requiem.

Ask Rostropovich about his musical family and he gives you an answer that is part open pride and part excited pleasure: "Galina is in London, singing five performances of 'Tosca' at Covent Garden. And the girls are again at the Juilliard School in New York City."

Ask him what else he is doing besides the National Symphony, and there is a long list of engagements, cello revitals, cello concertos with orchestras, and recordings. Opera? "No, no opera," he says. "For a while I do not conduct opera in opera houses. Stage directors are sometimes in the way. Next year, here, though" and what has become a kind of Washington look comes over his face." I would like to give with orchestra some opera in concert. When a small bird very quietly whispered the name "Iolanta" onto the passing breezes, a special look of possible pleasure crossed the conductor's face at the thought of the Tchaikovsky opera.

He had also some words about the programs he has announced for this first season. "I have studied very carefully the records of the audiences in the last few years, and they go down. I want to build an audience. And I want to build the orchestra. When I have audience, they will be with me when I play some new music."

Ask this man who has already added his talents to many dozens of recordings, both as cellist and conductor, if he has plans to record with the National Symphony and his answer is very firm. "Absolutely. But we do not rush around to companies and ask them to make records of National Symphony. When we are ready, they come to us." (THe hottest rumor in this area has been going around for a year or more: that the National Symphony's first recording will be a Tchaikovsky Violin Concerto with Isaac Stern as soloist, and Rostropovich conducting. That's not a bad combination when it comes to popular successes. But the contracts have not yet been signed. Any day now.)