The working relationship between an orchestra and its conductor is a close one, and yesterday Mstislav Rostropovich and the National Symphony gave the first hints of how close theirs will be.

It has been 2 1/2 years since Rostropovich, the famous Russian cellist, signed on as the fourth music director of the orchestra, succeeding maestro Antal Dorati.

From the evidence of his first rehearsal, which began promptly at 10 a.m. yesterday in the Kennedy Center, the NSO can expect to see its new conductor coax and cajole, demand and berate, plead and persuade as he works his will in the intimacy of the rehearsal hall. And the orchestra will respond or demur, balk or agree.

"I would like only to say how I'm proud to be here with you." Rostropovich said as he opened the session, having moved slowly to the conductor's stand, followed every moment by a battery of cameras, after a ceremonial kiss for the concertmaster.

Even before a note had been played, a major change was apparent: Rostropovich has reseated the strings on his right hand. The violas are now seated along the edge of the stage, with the cellos flanking them inside, a seating Charles Munch used in his years with the Boston Symphony. It is a classic arrangement, but one that few conductors have used with the National Symphony. It is intended to give the viola tone a desirable reduction in the quantity or quality of cello sound.

Then he brought down his baton to begin the Seventh Symphony of Dvorak, the work that closes his first subscription concerts next week, and for the next hour and 25 minutes began to shape the music and the players in his own way. His mood - and the mood on the stage - was buoyant.

Looking comfortable and at ease in a dark gray suit over an open tunic shirt, Rostropovich, using his steadily improving command of English, talked the working language of conductors everywhere:

"Intonation more precise" to the upper strings was quickly followed by "very good." A few moments later, "Energy is lost here." After a bit he returned to the problem of playing in tune - "Intonation not absolutely perfect, but after two weeks vacation . . ."

Rostropovich, it is now apparent, likes to stop as soon as anything needs adjusting or improvement. As the first movement continued, he paused at one place to say, "I would like, flute, a rubato here - yah-pah-pah-pah-pay - and the F sharp so lovely," suggesting to first flutist Wallace Mann that he could take a tiny bit more time and shape his solo phrase more expressively, rather than keep it strictly in time.

As a repeated phrase came long, Rostropovich stopped to explain to the strings, "In a place like this, one time you play the theme without force; the second time, play now with force." When the violins came to a sudden drop in dynamics, the great cellist's hands spread wide apart as he said. "Here very great change, from forte to piano," showing, by the distance between his hands, the switch he wanted from loud to soft.

The Dvorak Seventh Symphony has been on NSO programs in the past 25 years, but not with the regularity of the Eighth and Ninth. At one of Dvorak's characteristic melodies, the conductor stopped and said to the woodwinds. "It is very important that your line" - here he broke into very accurate singing to demonstrate - "must be more doice and more line.It is a beautiful song." That is one way of asking for a sweeter tone and a very sustained melodic line.

Then, since there are usually several dozen things going on in an orchestra at any one time during a big piece like the symphony, he turned to the violins:

". . . And while they are doing that, you must be more, um, undercover."

Rostropovich's increasing English vocabulary often becomes picturesque as he works to give his musicians descriptive suggestions. When a sudden look of disappointment flashed across his face, he turned to the violins and said, "It must be more like cream . . . not water."

And then, slightly accusingly: "Somebody will not play beautiful 16th notes - each note should be like a treasure."

Then, reasurringly: "Once more try. Do absolutely best."

"Absolute" and "absolutely" are two favorite Rostropovich words, since they suggest the utter perfection he is constantly seeking. Nothing routine was allowed to pass without correction yesterday.

At one point his voice rose over the orchestra as he reminded the violins to play "every note like a beautiful flower." Purely technical problems were grappled with as directly as subtler expressive matters. "No, no," again to the violins, "change stick or position. I hear 'yah dee dah dee dah dee dah DEE!' That's not beautiful." His request was for a change from one string to another, or a switch in bowing from up to down, but without the unwanted emphasis.

The clock at the back of the stage read 11:25 as the end of the first movement was reached. "Thank you, intermission," the new boss told his players. He was slightly more than half through the first of the two rehearsals he had called for his first day.

Ten minutes later, with the musicians again in their places, managing director Oleg Lobanov cheered them with the news that subscriptions are now at around 19,000, compared to last season's 16,100, with more coming in each day.

Once more at work making music, Rostropovich asked the orchestra for a "diminuendo with expression - not like when you turn radio down because it is too late for neighbors."

During the slow movement of the symphony, the strings were playing pizzicato - plucking the strings - in accompaniment to a clarinet solo. The conductor stopped them with a smile and said, "You must not play 'plunk, plunk' and not only look at me, but you must listen to clarinet," as he gestured toward Loren Kitt, playing first clarinet somewhat back in the orchestra.

No orchestra rehearsal is ever a cut-and-dried matter of musicians simply sitting there and playing the notes in front of them. It is always a time when, together, conductor and players come as close as possible to the way the conductor thinks the music ought to go.

Yesterday, as Rostropovich was working through the third movement, he stopped to say, "for this movement, you must have a special sense of sound. You play this like a popular song - but that's cheap. Play it like -" he stopped and turned to cellist Franz Vlashek with whom, at times, he exchanges his Russian for Vlashek's English equivalent.

After a moment Vlashek suggested, "remembered." "Ah, yes, Yes," Rostropovich beamed, and turning again to the violins finished his thought: "You must play it like remembered sound. Not 'I LOVE YOU' but," dropping voice to a murmur, "I love you."

The hands of the clock were inexorably moving toward 12:15, the time this rehearsal, by prearrangement, had to end. Personnel manager David Bragunier moved through the orchestra to give Rostropovich the required word - to remind him that time was up.

Before Bragunier got there, the conductor's left hand rose, pointing to the clock.

But his right hand continued to lead the orchestra in the closing beats of the movement - and at 12:15, right on the nose, the music and the rehearsal ran out.