The brother of imprisoned Soviet human rights activist Anatoly Scharansky said today that, for the first time in nine years, it appears possible that his brother's release is imminent.

Leonid Scharansky said, however, that while the most recent letter from Anatoly had mentioned improved conditions, it gave no hint that he expected any dramatic change in his status.

Published reports over the weekend said that U.S. and Soviet negotiators have reached agreement on an exchange that would allow Scharansky, one of the most prominent dissidents still held by the Soviet Union, to go to the West.

[Reagan administration and congressional sources said today that intensive negotiations have been under way in recent weeks for release of Scharansky as part of an extensive East-West swap, but officials would not confirm reports that such an exchange is imminent.]

In a letter from a labor camp in the Urals, Scharansky told his brother that his conditions had changed for the better.

"I don't know why. I don't try to think about it," the prisoner wrote in the 30-page letter that arrived Jan. 6, one of only three to reach his family in Moscow since last April.

In the letter, Scharansky briefly mentioned that he was allowed to read more, walk more and was given more chance to rest.

Today, as he assessed reports of his brother's rumored release, Leonid Scharansky allowed himself hope for the first time in nine years.

"This time I believe it is possible. This is the first time I believe it," the prisoner's older brother said in an interview. "There were rumors before and I hoped, but I didn't really believe them."

The reports of Scharansky's pending release to the West, nine years after he was first arrested, heartened members of Moscow's battered human rights movement, but they held their hopes in check.

"Many times such information has been broadcast and with no results," said Alexander Lerner, who for 15 years has been waiting for an exit visa to leave the Soviet Union for Israel. "We must be very careful with such hopes. So many times we have been disappointed."

Lerner was among the first to hear the report broadcast last night on the Russian-language service of the Voice of America. Like others, he noted that Scharansky's release would not necessarily augur changes in the Kremlin's policy on emigration, treatment of prisoners, tolerance of dissent or other human rights issues.

"I don't think my chances to leave will become higher after his release," Lerner said.

"One must be glad for Anatoly as a man, but this has no relation to the observance of human rights, or to emigration in general," said Naum Meiman, who also has been refused permission to emigrate.

Under Mikhail Gorbachev, the Soviet leadership has given mixed signals on human rights in recent months, as the Kremlin has pursued better relations with the West.

Before the U.S.-Soviet summit meeting in Geneva last November, permission was granted to Yelena Bonner, wife of Nobel Peace Prize winner Andrei Sakharov, to go abroad for medical treatment. Visas also have been granted to a group of Soviets married to Americans, leading to the reunification of a half dozen divided families.

But at least a dozen similar cases remain, with Soviets denied permission to join foreign spouses for a variety of reasons, some never explicitly stated. Despite flurries of rumors about a new loosening of Jewish emigration, the number of exit visas -- 1,140 last year, compared with 51,000 in 1979 -- has not risen significantly.

The mixed picture suggests to some that the Kremlin is using isolated cases for maximum public relations effect in the West. Many activists say they think any real relaxation on Jewish emigration will come about only through political bargaining. "They will never do it for nothing, only if they can gain something," Lerner said.

The case has held special significance for Jewish and human rights activists here.

At the time of his arrest in 1977, Scharansky, now 38, was a central figure in the dissident movement, as a spokesman for "refusedniks" -- Jews refused permission to emigrate -- and a member of the Helsinki Watch group, formed to monitor human rights abuses in the Soviet Union but since disbanded.

Scharansky, a mathematician fluent in English, was charged with espionage and treason and convicted in a closed trial in 1978. He insisted that he was innocent but was sentenced to 13 years -- three in prison and 10 in a labor camp. The prison sentence was extended three years after he was accused of being a bad influence on other prisoners.

President Jimmy Carter made Scharansky's case part of his campaign against Soviet human rights abuses, issuing limited economic sanctions in retaliation.

Scharansky's wife, Avital, emigrated to Israel in 1974 just days after their wedding. She has attended virtually every important meeting involving Soviet leaders in the West, pressing for the release of her husband.

As Scharansky has become one of the best known prisoners in the Soviet Union, his name reportedly has been mentioned regularly when Soviet human rights abuses are brought up in high-level Kremlin meetings.

"They feel Scharansky's case has given them a lot of trouble," Lerner said of the Kremlin officials. "Maybe they have decided it is enough."

Before his visit to Paris last October, Gorbachev was asked about Scharansky by French television reporters. In an answer, broadcast here and in Paris, he put the case in the context of "people who by some logic or another are at odds with the Soviet form of government, with socialism, and profess some different ideology."

What happened to Scharansky, Gorbachev said, was that "he broke our laws and was sentenced by a court for that."