The insistent question underlying final negotiation of an Israeli-Lebanese agreement on withdrawal of Israeli troops from Lebanon is: ''Will the Syrians accept it?''
Until US Secretary of State George Shultz arrives in Damascus on Saturday, no one can be fully certain of the answer.
Secretary Shultz has called this question a ''separate matter'' for settlement between the Syrian and Lebanese governments. But, as he acknowledged, without a Syrian troop withdrawal as well as a pullback by Palestine Liberation Organization troops remaining in Lebanon, the Israelis will stay put as well.
Certain facts are already clear. Syrian Foreign Minister Abdel Halim Khaddam, who met Mr. Shultz on two occasions in the United States late last year, reportedly told the secretary that Syria was prepared in principle to withdraw.
On the other hand, top Syrian officials have repeatedly stated that they will not accept any military, political, or economic ''rewards'' for Israel in an agreement. Syria regards Israeli troops in Lebanon as aggressors while recalling that Lebanon invited Syrian troops in 1976 to intervene in the Lebanese civil war. This mandate expired in June 1982.
''Israeli troops must withdraw from all Lebanese territory unconditionally,'' said Syrian Information Minister Ahmad Iskander Ahmad in an interview.
Yet informed diplomatic observers agree that any Israeli-Lebanese agreement will inevitably have many aspects of a peace treaty. Syrian President Hafez Assad visited Saudi King Fahd on Thursday in what Israel radio termed ''an apparent attempt to get him to oppose an agreement.''
The Syrian decision will be based on a number of factors. One major concern is their own security. Syria's troops sit mainly in Lebanon's Bekaa Valley, which is ethnically and geographically close to the Syrian heartland. They control the road to Damascus. Israeli troops facing them are only 15 miles from the Syrian capital.
''If Syria could maintain influence in Lebanon without keeping troops there, she would prefer it,'' said one Western source in Damascus. ''But if Syria would lose influence or if its security were threatened, it wouldn't leave.''
US efforts have been focused on putting together a package of security guarantees for south Lebanon that would satisfy Israel but appear the least threatening to the Syrians.
''The key is the way the package looks in its entirety,'' said one source in Damascus. This strategy has been behind US efforts to get Israel to accept a downgraded status for its ally, south Lebanese militia leader Maj. Saad Haddad, whom both the Lebanese government and the Syrians view as an extension of the Israeli military.
But Damascus radio said Thursday that Syria would do all in its power to see that the agreement was not implemented. The radio said Syria opposes ending the state of war between Israel and Lebanon before a complete Israeli withdrawal to the Israeli border. The Syrians also oppose any special security arrangements for Israel in south Lebanon.
Syrian willingness to accept an agreement must also be influenced by the Syrians' estimate of the cost of remaining in the Bekaa. Economically, the burden is partly offset by money from the Arab states, an annual appropriation of $1.8 billion. However, Algeria and Libya don't pay, cutting out about $400 million. In private Syrian officials admit that some Gulf states are slow to send funds.
However, informed analysts in Damascus believe that on such an issue the decision will not be made on economic grounds. Nor will Syrian President Assad be hampered by internal dissent, as analysts here agree that such dissent has been well under control since the government smashed a revolt by the fundamentalist Muslim Brotherhood a year ago.
On military grounds, some of Syria's PLO allies have pushed for staying on in Lebanon in order to tie down Israeli troops. PLO leaders would dearly like to keep their remaining bases in Lebanon.
Against this must be measured the threat of war in the Bekaa if Syrian and Israeli troops remain face to face. Syrian leaders are convinced that Israel, backed by a US green light, will eventually launch an attack if Syrian troops remain in Lebanon. Yet conversations with Syrian officials and informed Western analysts indicate that the Syrians - rearmed by the Soviet Union - will risk war rather than accept an agreement they dislike.
As for the Soviet dimension, some Western analysts believe that Soviet influence would not deter Syria from an agreement that it favored. But if Syria views an Israel-Lebanon accord as an attempt to expand US and Israeli influence in the region, then its opposition to such an agreement is likely to parallel that of the Russians.
Moreover, some Syrian officials have clearly indicated that any Syrian agreement to a troop withdrawal plan will hinge on the broader question of an overall Mideast peace plan. Syria has opposed the Reagan Mideast initiative, which gives no role to Syria's PLO allies and rules out an independent Palestinian state. The Syrians have long favored an international peace conference with Soviet participation. This in the end may be the hardest issue for Mr. Shultz to deal with, and the trickiest to avoid.