"For a year," says Mstislav Rostropovich, "I have not been sharing music with my friends in Washington, with my orchestra. I need that now. I am so eager that when I come back, I will jump on the stage."

Rostropovich took the whole of 1984 as a sabbatical from the National Symphony Orchestra's subscription concerts. His official return will begin at 4:50 p.m. tomorrow, his scheduled landing time at Dulles Airport (with a substantial welcoming committee and miscellaneous fans on the scene to greet him). He will then begin rehearsals for his first concert (including a world premiere) on Thursday evening. In his absence, this account of how he spent his sabbatical year and what he plans for the future has been pieced together from conversations during his visits to Washington in the past year.

"There is not enough time," he says. "Too much to do." Sabbatical or no, it was hardly an uneventful year, musically or personally.

He has played the cello around the world, including two benefit recitals in Washington. He has particularly emphasized the Bach Suites for unaccompanied cello, which he plans to record "not yet; but nearer and nearer; I work very much."

He has done some conducting. In Washington, he conducted the Catholic University Orchestra and Chorus in a memorable interpretation of Rachmaninoff's "The Bells." In a Stuttgart concert broadcast to 30 countries, he conducted the first complete performance of Penderecki's "Polish Requiem," sections of which he has already conducted here. At Aldeburgh, England, he directed the second annual Rostropovich Festival, begun as a tribute to his friend Benjamin Britten, who lived in that English seaside town and held a summer festival there each year.

Next summer, at Aldeburgh, he will conduct the first performance of Britten's unfinished cantata, "Praise we great men," which was being composed for him at the time of Britten's death in December 1976. Originally, "Praise we great men" had been planned for Rostropovich's first concert as music director of the NSO. The text is based on a poem written by Edith Sitwell in honor of Britten's 50th birthday, and the composer had finished 11 pages in piano score at the time of his death.

He became one of the leading characters (along with Dmitri Shostakovich, Nikolai Bulganin, Aleksandr Solzhenitsyn and a host of others) in "Galina" -- the autobiography of his wife, soprano Galina Vishnevskaya, and one of 1984's most enthusiastically received books about music.

"Now," he says, "I am better known as a husband than as a musician. People stop me in airports and ask me to sign the book: 'Husband of famous author.' It was written to explain to our new friends what it is like living under that other system -- to give a human example of what communism means." Of the sometimes vivid descriptions of himself in the book, he says, "I did not try to change a word." As if he could have.

His friend, pet and international traveling companion, a little dog named Pooks, died of a heart attack. "I received many letters of sympathy for Pooksie's death," he says. "Now, I have Mukha, who lived with Pooksie for a year and was her friend. Mukha plays the piano like Pooksie; I have bought her a small piano and I take her on all my tours." She was given the name Mukha (which means "fly" in Russian) because she is small and black. In spite of her musical instincts, it has been decided that she will require more training before she is allowed to attend NSO rehearsals.

In Venice he was awarded the Vita nella Musica prize. "This is not given very often," he observes. Only five people have received it: Arthur Rubinstein, Carl Bo hm, Andres Segovia, Yehudi Menuhin and I. That puts me in very good company." In London, he was made a fellow of the Royal College of Music.

Meanwhile, in Washington, Rostropovich has hardly been an absentee music director. Besides conducting the NSO on a wildly applauded Latin American tour and in a standing-room-only July 4 concert at the Capitol, he has been quietly slipping into town and listening to his orchestra perform under other conductors.

"Usually, I have no time to listen like a member of the audience," he says, "and this is very important. I listen from all parts of the Concert Hall, and I ask myself what can be done to improve the sound."

He has also been acting like a member of the orchestra. "I have been sitting in at rehearsals," he says, "taking different positions so I learn how each musician hears the other musicians when they play. Some people complain, especially those who sit in front of the brass and percussion; sometimes they get noises like a bomb. When I sit in the first violin section, I don't hear one sound from the violas."

Standing on the podium and waving a baton is only a small part of a music director's job, and it is the only part from which Rostropovich took a sabbatical last year. He has visited Washington regularly to meet with the musicians, and in November he gave a party in his apartment near the Kennedy Center for 15 members of the orchestra who have joined it in the past few years. "I must meet with the musicians not simply as a conductor but as a human being," he says. "This is not just an orchestra; it is my friends, my family."

With each passing year (this is his eighth with the NSO), it becomes more and more his orchestra -- musicians he has personally auditioned and hired have been joining those he inherited from his predecessors, Howard Mitchell and Antal Dorati. During his "absentee" year, eight string players hired by Rostropovich joined the NSO: three violinists, one violist, two cellists and two bassists. The care with which he makes his choices can be heard in the orchestra's constantly improving sound. It has also become his orchestra in the sense that it reflects his approaches to music more easily and spontaneously; communication becomes easier with the musicians and, through them, with the audience.

How does he feel when he hears someone else conducting his orchestra? Mostly, he says, the feeling is pride; sometimes he picks up useful information to be filed away for later use -- how the audience hears the music and how other conductors handle the orchestra.

"My job this year, listening, was to establish clear goals for the orchestra's sound, balance and style, to find the right mixture of technical skill and emotional power," he says. "Sometimes, when you put too much emphasis on technique, the emotional strength disappears.

"I hear different kinds of conductors, some very exciting, some with great technical ability. Fru hbeck has both; he makes everything so clear. I was proud of our cello section when he conducted the Verdi Requiem; they played with such phenomenal intonation. I did not go back to hear it again. I thought: 'Maybe they will not be so perfect the next time.' "

When he started with the NSO eight years ago, Rostropovich and the orchestra both seemed to need seasoning. As a conductor, he had not built a reputation anywhere near what he has as a cellist. He still hasn't, but the gap is a lot less notable.

He cultivates nuances he seldom used to bother about between the extremes of fortissimo and pianissimo, between very fast and very slow tempos. He has gradually mastered the art of balancing an orchestra's sound, and he has expanded his repertoire, which was once strongest in Russian music and music composed by personal friends such as Penderecki and Britten. His Beethoven, which was sometimes eccentric, has not become definitive, but it is respectable, and he has begun the slow conquest of 18th-century styles (Mozart, Haydn, Bach and Vivaldi), which seem the most difficult for musicians trained in the Russian tradition.

The orchestra has grown steadily along with its conductor. It is certainly not yet in the American Big Five, but on occasional good nights it can equal any one of them, and on the average it probably ranks among the top dozen. Rostropovich's sabbatical was taken as a time for reevaluation in the middle of what looks like a long-range growth process. How long it will continue and where it will end is anybody's guess, but he has built up a momentum in Washington that should keep him here at least through the '80s and perhaps far beyond. If the growth continues at its current rate, the final results could be spectacular.

Audiences can expect Rostropovich's return to launch a series of experiments in the seating of the orchestra. "There are three or four different possibilities," he says, including rearrangement of the orchestra's sections and possibly the use of risers.

His 1985 season will begin with a bang -- specifically with the world premiere of "RiverRun" by Stephen Albert, a purely orchestral work inspired by James Joyce's "Finnegans Wake" and commissioned by the Sidney L. Hechinger Foundation.

Albert will share the program with Beethoven: the "Fidelio" Overture and the "Pastoral" Symphony. "I think that's a masterpiece," he says of "RiverRun." "Stephen Albert is a very young composer and extremely talented." His second program will feature a repeat performance of Ezra Laderman's Symphony No. 5 ("Isaiah"), for which he gave the world premiere in 1983. On the same program, he will be joined by Jean-Pierre Rampal in Bach's Second Suite for Orchestra. The following week will include scenes from Mussorgsky's "Boris Godunov," with the Oratorio Society and bass Matti Salminen -- a reminder that Russian opera is one of Rostropovich's specialties too seldom heard in Washington.

It will be heard more extensively in the 1986-87 season when he conducts Rimsky-Korsakov's "The Tsar's Bride" for the Washington Opera, with Vishnevskaya as stage director. "This is a new opera for the United States," he says, "a great opera. We will do it in a traditional way. Experiment is all right, but not when you are doing a masterpiece for the first time. I am very happy that Galina will be directing. Who has more experience than she? Nobody in the West."

Other long-range plans include a tour of the Midwest and Canada in March, and possibly a European tour (if money can be raised) in September. The 1985-86 season will be announced at the end of this month, and plans are already well advanced for the following seasons. "In 1987 will be coming together my 60th birthday and my 10th year with the symphony," he says. "I will repeat some of my most successful efforts from the first 10 years, and we hope to have many distinguished guests. We are talking to Bernstein, Ozawa, Leinsdorf, Tennstedt. Then, in 1988, I am planning an international cello congress in Washington."

Besides his Washington home, Rostropovich has apartments in London and Paris and a large estate in upstate New York. Deprived of his Russian citizenship, he has become a citizen of the world, traveling on an international passport.

"But," he says, "in the whole world, if you ask me which is my town, that's here -- Washington."