A top official of the United Nations has just returned to New York from the Middle East with a healthy and heartening antidote to the mind-numbing Big Picture of the recent problems of war and peace.

What U.N. Undersecretary-General Brian Urquhart discovered in a few days of quiet diplomatic probing in Lebanon, Syria and Israel is practical, down-to- earth stuff: the existence of a real possibility of breaking the deadlock over a large part of the problem of Lebanon. And what's interesting about Urquhart's sense of new opportunities is the various ways in which they would appear to be at odds with some fundamental assumptions in the Reagan administration.

Never mind the president's ringing pronouncements at the General Assembly. Administration officials have made no secret of their disenchantmen with the U.N. in general and with its potential for peace-keeping in Lebanon in particular. Until very recently, the administration has accepted Israel's insistence that its withdrawal from Lebanon be linked to simultaneous Syrian withdrawal. The administration has dismissed Syria as a villainous adversary, a Soviet surrogate, a prime source of "state-supported" terrorism. It insists that American influence as an honest broker in Middle East conflicts remains undiminished despite its failed mission in Lebanon.

At least one of these assumptions has already been overtaken by a statement from the new Israeli foreign minister, Yitzhak Shamir. It effectively removes any strict condition that Israeli withdrawal be matched by Syria. Now it is true that Shamir has invited the United States to play the mediator's role and the United States has followed up on Urquhart's initiative with extensive soundings of its own. Shamir and U.S. Secretary of State George Schultz are pursuing the matter in their talks this week.

It is not yet clear whether the U.S. or the U.N. -- or both -- will do the mediating. What is clear is that an expansion of the U.N. peace-keeping role in Lebanon, filling in behind Israeli forces as they withdraw, would be the key to any agreement. And the reason for this has much to do with the new influence of the labor party in Israel's new government of national unity. According to Israeli and U.N. officials in a position to know, Yitzhak Rabin, the former Labor prime minister now serving as defense minister is strongly in favor of heavy dependence on the U.N. Shamir, on the other hand, reflects a longstanding Israeli distrust of the United Nations dating back to its abject performance at the outset of the 1967 Six Day war.

Ideally, even the Labor Party members of the new government would probably share the Likud's preference for a combination of regular Lebanese army units and an Israeli-trained "South Leba non army" to police southern Lebanon and ensure that it will not once again be used as a base for rocket and artillery attacks on northern Israel. Such attacks, by forces of the PLO, provoked the 1982 Israeli invasion of Lebanon.

There is not much evidence, however, that any combination of Lebanese forces would be ready any time soon to provide reliable security in the south. Meanwhile, both Rabin and Prime Minister Shimon Peres are plainly in a hurry. The Israeli occupation in southern Lebanon is costly.

Syria's wishes, moreover, must be reckoned with. The Syrians are on record against U.S. mediation and apparently in favor of a substantial U.N. role. Any enlarged U.N. role, however, means a bigger voice for the Soviet Union.

At least some Israeli leaders are ready to accept the risk of a wider Soviet role. They even sense an important change in Syria's willingness to reach some sort of accommodation with the United State, in the interests of advancing Syrian objectives in Lebanon. Not the least of these objectives is Israel's withdrawal. Some Syrians apparently hinted at an interest in striking a somewhat different balance in their respective relations with the United States and the Soviet Union for the sake of easing their dependence on the latter. The dispatch of U.S. Assistant Secretary of State Richard Murphy to Damascus is said to be at least partially in response to precisely such hints.

Hence the problem now confronting Reagan and his close associates: on the one hand, there is the Big Picture they have been presenting to the world of the Soviets as the source of Syrian evil and of the Syrians as the source of anti- American terrorism. On the other hand, there are the new Israeli interests and the new Syrian inclinations reflected in the findings of Urquhart's peacemaking initiative.