On the eve of talks here between the Soviet and West German foreign ministers this weekend, the Warsaw Pact powers now meeting in Sofia are expected to pronounce a ''collective response'' to NATO's pending placement of new nuclear missiles in Western Europe.
Andrei Gromyko, the Soviet foreign minister, is in the Bulgarian capital for policy sessions Thursday and Friday with his allied counterparts. He will then fly to Vienna for talks Saturday and Sunday with West German Foreign Minister Hans-Dietrich Genscher.
Buildup to the Soviet ''response'' has gathered considerable momentum through this week, both in statements by various East European leaders - notably East German and Czechoslovak - and by Marshal Viktor Kulikov, the Warsaw Pact's commader-in-chief. Kulikov said Tuesday that ''all necessary measures'' would be taken to maintain what the Soviets see as a present East-West weapons balance.
This parity, the Russians insist, will be undone by the placement of new United States intermediate-range missiles in West Germany and four other NATO states. The cruises and Pershing IIs are due to be deployed starting in December if Moscow has not agreed to cut its SS-20 missile strength targeted on Western Europe.
According to one well-sourced report, the Soviets plan to respond to NATO deployment by adding a new generation of rocket-powered SS-21s, 22s, and 23s, as well as the SSCX-4 jet-powered missiles, to the Warsaw Pact's armory.
The SSCX-4, a sea-based projectile with a range of about 900 miles, is mentioned as one of three measures Moscow reportedly has in mind. The other two measures are to end the present moratorium on the initial SS-20s, and to station tactical nuclear rockets in several East European countries.
Soviet military sources are quoted here as claiming that the submarine-carried SSCX-4 is capable of reaching any part of the US. For some time, East European sources have been saying the Soviets are leaning toward positioning the SSCX-4 in the sea rather than in East Europe.
The Genscher-Gromyko talks are expected to bring some clarification on points like this. They should also address a series of recent conflicting reports on the Soviet Union's likely attitudes and intentions in the impasse with the US.
The Vienna meeting is especially important since at this point in East-West relations, Bonn is the only NATO capital with substantial lines of communication still open to Moscow.
West Germany has been the Russians' special military obsession and political target in more than a decade of arms reduction negotiations and East-West ''coexistence.'' They give West Germany such attention both because of their World War II experience and because West Germany now maintains the largest of NATO's forces in Europe.
West Germany has its paricular interest in the relationship, too. As Mr. Genscher said in press interviews this week, ''we would be the hardest hit'' if the US-Soviet arms talks fail and all that such a failure might entail.
West Germany, he said, is not directly involved in the Geneva negotiations. But, he pointed out, ''Our national interests render it imperative for us to do everything possible to make the talks a success.'' And that will be one of his likely leitmotifs in his efforts to persuade Mr. Gromyko of the sincerity behind West German efforts.
For his part, Gromyko will obviously be making a strong bid to persuade Bonn to at least delay its acceptance of the new weapons pending further negotiation. The Soviet Communist Party daily Pravda charged this week that their stationing would, in fact, breach the friendship treaty between East and West Germany signed in 1972, and would therefore be tantamount also to a threat of force.
Genscher is expected to refute such arguments firmly. But he is also likely to emphasize to Gromyko West Germany's serious concern over maintaining East-West contacts, and insist that negotiations with the US must continue even if the deployment begins.
Underlining the proxy role West Germany is playing in the current Geneva standstill is the presence of a large group of Bonn parliamentarians in Moscow this week and of a close Kremlin adviser, Leonid Zamyatin, in the Federal Republic.
Both Zamyatin and the German parilamentarians, in their comments to the press , said they could see no alternative to at least an interruption of the Geneva talks if NATO goes ahead with deployment.
But the authoritative word may be spoken here by Gromyko this weekend. It still seems plausible that the Soviet Union will be in no hurry to close doors while there is still more time to continue exerting pressure both on West Germany and Western Europe at large.