THE MEDVEDEV brothers have been one of the remarkable phenomena in recent Soviet history. Roy, living in Moscow and talking continually with diplomats and correspondents, has defined the way that Western and Soviet publics have seen the pattern of Soviet politics over the last four years. Zhores, in emigration, has presented a similar view abroad.

Roy Medvedev is a democratic socialist who has managed to publish many articles abroad and to occupy a semi-dissident position while barely staying within the law. In the words of an American diplomat in Moscow, "he Roy Medvedev has an explanation for any political event." Since he has been the only man in the closed Soviet system willing to do this consistently, he has been the main source of rumors and speculation about high Kremlin politics.

Those in Moscow skeptical about his reliability -- and they are legion -- often accept his basic story, for want of anything else. Moreover, Medvedev's rumors and speculation are broadcast back into the Soviet Union by foreign radio. When they are picked up by other Soviet citizens from this source, they seem to receive confirmation through repetition.

Zhores Medvedev's biography of Gorbachev, like his earlier biography of Andropov, tends to follow the general interpretation advanced by his brother. Over half of the book deals with Gorbachev before he came to power, but the sparsity of data requires Medvedev to use a good deal of filler material about the institutions in which Gorbachev lived and worked, as well as about higher Politburo politics of the last 10 years as Medvedev sees them.

The treatment of Gorbachev's early life is balanced, but the driving forces of the man do not emerge. Why in heaven's name did a bright and ambitious boy, capable enough to be admitted to the elite Moscow University, choose a low-status law career in 1950 instead of engineering -- the normal path of upward mobility into administrative and political work? What was the impact of rooming for five years with a liberal Czech who was to become a Politburo member under Dubcek? How did Gorbachev see the Khrushchev de-Stalinization, which was the great political event of his twenties? The answers are not in this book, but this is not Medvedev's fault. We simply do not know.

The book's view of Gorbachev of the 1980s will be very familiar to the layman who has been following Soviet events in the media. Gorbachev is described as a prote'ge' of Andropov and Suslov in their struggle against the Brezhnevites. The earlier Medvedev image of Ustinov as the dominant number-two man under Andropov has, however, quietly been replaced by an interpretation that properly gives much more weight to Gorbachev in Andropov's year.

The Gorbachev of the last year will be completely familiar. Gorbachev is a man who wants to get the country moving, but his economic strategy is "constructed along traditional lines"; he opposes any liberalization; the chief members of his team (Viktor Chebrikov, Yegor Ligachev, and Nikolai Ryzhkov) are "orthodox and could even be called hard-liners"; there is new diplomatic activity, "but the same old foreign policy." Yet, for some reason there is "a distinctively new era in Soviet history." Gorbachev may not have made his final choice, and change is necessary.

TO REPEAT, it's familiar. Nevertheless, the book leaves a very uneasy feeling. It is quite detailed on the institutional and political level, but the detail is often flat wrong: (Gorbachev is said to have skipped the position of regional third secretary in his rise, but there is no such post; the Leningrad city soviet is not subordinated to the Leningrad regional soviet; the central committee secretary for ideology does not have the control over most personnel decisions).

Sometimes the error is an important link in interpretation. Usually reliable Kremlinological indicators suggest that Gorbachev originally was close to Chernenko. In arguing that Chernenko was trying to undercut Gorbachev, Medvedev says that he had the Moldavian Ivan Bodyl appointed as first deputy prime minister in charge of agriculture. In fact, Bodyl was deputy prime minister, not first deputy, and he had nothing to do with agriculture, but was in charge of trade.

These examples are quite verifiable, but what are we to say about the many unsubstantiated rumors that are cited as fact (e.g., that Bodyl was Brezhnev's candidate for Central Committee secretary for agriculture, and Gorbachev was selected in 1978 over his objection)? No one can say, but these rumors must be treated with extreme care.

And what is to be said about the basic interpretation of Gorbachev when one side of reality is not being given? One can say that Gorbachev is not politically liberal, but, in fact, a major expansion has occurred in the definition of what can pass the censor in the media and culture. Moreover, Gorbachev has been extremely innovative in his ideological formulations and revisions and extraordinarily insistent on "radical reform." He has ridiculed those who are for transformation, but think that this can be done without change. "This will not be, comrades," he said. In foreign policy he has given signal after signal that he plans to abandon an American-centered policy and focus on Europe and Japan.

To be sure, actual economic and foreign policy are still only featured by marginal change, although substantial in comparison with Reagan's first year. Maybe this will continue. Maybe. But, if so, Gorbachev is a total fool to have raised expectations so high by overpromising on reform. If Gorbachev is going to continue the American-oriented policy, it is strange that both his new first deputy ministers of foreign affairs have a European background, that the deputy minister of foreign affairs for the United States has the lowest status of all the deputy ministers, and that the new ambassador to the United States is of lower status than all the major ambassadors to Europe and is a Europeanist who hardly speaks English.

If it does turn out that Gorbachev's words matter, we should begin to think more seriously about our wisdom as a country in relying so heavily on one source of interpretation for Kremlin politics. And, maybe, Soviet leaders should ask themselves whether it has really served their interests to remain so close-mouthed about political matters. There are benefits to a freer information policy in all spheres.