After 2 1/2 years of virtually nonstop mutual hostility between the Reagan administration and rulers of the Soviet Union, a series of small but "mildly encouraging" developments, as one senior U.S. official put it, have rather suddenly sprouted in the relationship between the nuclear giants.

Specialists throughout the administration stress that these developments are on the fringe of that relationship and that central hostilities and differences remain. But there is a feeling that the new developments may amount to at least "a clearing away of the underbrush" from a road that could lead to better relations.

"I don't want to make too big a thing out of it," one high-ranking and normally hard-line official said of arms-control developments. "But both sides have lifted their skirts a little and are showing more ankle. What we don't know on the Russian side is if it is really a signal that they want to talk, if it's a tit-for-tat response to our efforts or if NEWS ANALYSIS it is something they would have done anyway."

In the past month or so, here is what happened.

In Washington June 8, President Reagan announced slight changes in the U.S. position at the Strategic Arms Reduction Talks (START) in Geneva that would in effect move the U.S. stand closer to the Soviet one at least on the number of long-range missiles allowed each side. Many officials also suggest that other areas of flexibility in the U.S. proposal have not been made public.

* In Geneva three weeks ago, the Soviets dropped some of their more extreme START demands, which they knew to be totally unacceptable to this country.

Two weeks ago, the Soviets made a more serious proposal, which leaves intact their biggest missiles that most concern the United States but which nevertheless indicates that Moscow is willing to move to lower overall multiple-warhead missile levels than it was in earlier arms agreements.

* In Vienna late last month, the Warsaw Pact put forward a new proposal at the long-stalled East-West talks on troop cuts in Europe.

As in the START negotiations, the Soviet move did not deal with the central issue threatening the West--the western contention that there are far more Soviet-bloc troops in Eastern Europe than Moscow admits. But the new proposal at least had "some positive elements," the White House said, in appearing to allow greater on-site inspection rights.

* In Warsaw, Polish authorities have absorbed last month's visit of Pope John Paul II and appear to be preparing to lift at least some of the remaining martial-law restrictions this month.

Administration officials believe that the papal visit went extremely well, and Reagan has stated publicly that if Polish authorities take actual rather than cosmetic reforms, the United States and its allies will consider lifting some or all of the economic sanctions imposed late in 1981.

Undersecretary of State Lawrence S. Eagleburger conveyed this message personally to a senior Polish diplomat here recently in a private meeting that was reportedly the most substantive held with Polish authorities in a long time.

* In Madrid, both countries have just signed a compromise agreement that ends nearly three years of talks at the 35-nation conference to review progress on the 1975 Helsinki accords on security and human rights in Europe.

The compromise is also expected to commit the Soviets to a follow-up conference on human rights and a new conference on disarmament in Europe meant to restrict the kind of military activities that makes opposing blocs worry about surprise attacks. It also will lead to a meeting this fall between Secretary of State George P. Shultz and Soviet Foreign Minister Andrei A. Gromyko.

* In Moscow June 26, the Soviets allowed 15 members of a Pentecostal family that had taken refuge in the U.S. Embassy for almost five years to emigrate to Israel. Another Pentecostal family that took refuge in the embassy has been told it will be able to leave the Soviet Union, the State Department said.

More recently, there have been reports that other dissidents will be allowed to emigrate this year. Whether this is part of a secret deal with the United States cannot be ascertained, although some U.S. analysts believe it is.

Private contacts between Shultz and Soviet Ambassador Anatoliy F. Dobrynin have also increased considerably in recent months. Just before Dobrynin left here Friday for his annual round of consultations and vacation at home, he met with Shultz. In these discussions, officials said, the United States has stressed its readiness for serious negotiations on fundamental issues.

Precisely what this catalogue of events means or where it may lead is something about which administration specialists will not speculate.

"The attitude is to look at the cards that are on the table," one top State Department official said, "rather than speculate about what's in Moscow's hand." Most interesting collectively about these recent developments, he said, is that they come as private contacts among Shultz, Dobrynin and others are increasing.

No one here really knows what is on the mind of Soviet leader Yuri V. Andropov, officials said. The fact that he has possibly serious health problems has made calculation about Soviet intentions doubly hazardous.

Most specialists said Andropov faces serious economic problems and wants to concentrate on domestic issues, suggesting that he may be seeking a respite from international competition.

But specialists also said that a main part of his support comes from the powerful military-industrial complex and that he thus may not be able to make real arms control concessions that cut into the kinds of weapons that Washington most fears and the Soviet military leaders most value.

As for what is on Reagan's mind, most administration officials contended that the United States is in a reasonably good position to negotiate and is receptive to serious efforts. The defense budget has been built up, work is proceeding on new MX and Trident II missiles, B1 and Stealth bombers and cruise missiles, allied unity is reasonably firm, the economy is improving and Reagan's political stock remains strong.

Officials acknowledged that the White House is under growing political pressure for an arms control agreement as an election year approaches. It also has only shaky confidence that it can keep congressional support for the new land-based MX intercontinental ballistic missile unless it shows consistent progress toward arms control. The administration believes that the MX is vital to entice Moscow to negotiate seriously.

Yet it is difficult to say with certainty whether the administration simply feels it is in good shape for negotiations or whether it can package a compromise defensible against attackers from the political right and left.

Moscow and Washington, however, are facing two potentially explosive situations that probably must run their course before any real progress on arms control, which remains at the center of the relationship although not the only measure of it.

Large demonstrations in western Europe are planned this fall by groups opposed to scheduled deployment in December of new U.S. missiles there to balance weapons fielded in the western Soviet Union. There has been no progress in the companion negotiations in Geneva on limiting these intermediate-range nuclear forces of both sides.

This fall, the Soviets are likely to submit a new proposal that will interest the protesters and tempt the allies' opposition parties to seek postponement of deployment while more talks are held.

The alliance, however, is committed to deployment on schedule if no agreement is reached by year's end. So the next few months are apt to bring great new strains as the Soviets play on western Europeans' concerns.

Similarly, some analysts believe that when initial deployment occurs Moscow will withdraw its existing START proposal, arguing as it has all along that the new U.S. missiles in Europe are "strategic" in that they can reach Soviet territory. The Soviets have always made their START proposal on long-range strategic arms contingent on no new western intermediate-range missiles in Europe.

If this crucial confrontation in December can be bridged, it is widely felt here that early and mid-1984 could provide a setting for fruitful negotiations.

In this view, the Soviets will not agree ahead of time to any compromise that would allow new U.S. missile deployments in Europe, because this would undercut western European groups who also do not want missiles.

Once the missiles are deployed, it is argued here, it may be easier to negotiate removing them or setting some joint weaponry level with the Soviets, perhaps combining the two sets of missile talks into one larger framework.

By late this year, it will also be clear whether Reagan is running for reelection, and the Soviets may have decided whether to negotiate with him as a candidate and possible second-term president or take their chances with a different U.S. leader.

Before then, another potentially explosive issue faces both sides. It concerns allegations of Soviet cheating on previous agreements on chemical and biological weapons and testing of new missiles and encoding the test data.

Many conservative senators, and one high-ranking official in the U.S. Arms Control and Disarmament Agency in particular, are pressing the White House to accuse Moscow openly of violations. A top-level White House group is studying this, and Reagan has not made such accusations.

The issue is difficult because the view is widespread, even among administration moderates, that the Soviets are skirting past agreements. But the wording of the agreements, in the missile area, is extremely vague.

Charges by the president might raise public questions about the value of future negotiations. Yet such value may be vastly greater than the risks of cheating on the margins. So the question is how big an issue to make of the situation.