The deployment of Syrian ground-to-air missiles in Lebanon, in full view of travelers and with uncharacteristic casualness, appeared to diplomats here today to be a carefully prepared message and challenge to Israel.

The message, according to diplomats, was that the positioning of the missiles by the Syrians Wednesday, a day after Israeli warplanes shot down two Syrian Army helicopters in eastern Lebanon, was a limited act, but conceived so as to inflict a high cost in aircraft if Israel attempted to destroy them.

This afternoon this reporter drove without hindrance through several normally suspicious Syrian Army checkpoints in eastern Lebanon and saw gyrating radar dishes and two groups of SA6 missile batteries mounted in threes fully deployed in highly visible positions.

As if Syria wanted to advertise their presence, two batteries were behind earthworks on the highly traveled Beirut-Damascus highway half a mile east of the junction with the road leading north to Rayak Air Base.

The other three-battery cluster was situated two miles north on that road.

Overnight on SA2 battery and an SA6 battery spotted yesterday by eyewitnesses along the Rayak road had been moved, but military specialists said it was unlikely that they had been taken back to Syria.

In an extensive drive through the narrow Bekaa Valley far north and south of the displayed weapons, there was no indication that other batteries have been stationed in Lebanon.

Specialists said the SA2 battery prevents Israeli aircraft from attacking via the Bekaa Valley, which leads north from the Jewish state, unless the Israelis are willing to lose aircraft over Lebanon for the first time since the 1973 Arab-Israeli war.

Combined with sophisticated air defense systems in Syria, the SA6s were said to provide protection from Israeli attack from other directions.

"The Israelis could come in a big saturation raid and hit all the Syrian batteries at once," a specialist said, "but they would have to be prepared to lose aircraft and pilots."

Losing aircraft and pilots is a risk Israel has not had to take seriously in Lebanon since 1976, when it tacitly allowed Syrian troops to enter and police this country, in exchange for several Syrian concessions that reportedly included keeping ground-to-air missiles out.

When Israel shot down two Syrian helicopters Tuesday, the Syrians had no way to reassert their damaged prestige, according to specialists, except the introduction of missiles, since Israel had gained such complete control of Lebanese airspace.

The SA2 is a cumbersome, 30-foot-long missile of the type that the Soviets used to bring down Gary Powers' U2 spy plane in 1960. The SA6, also Soviet-made, is a newer missile, used by the Egyptians against Israel with deadly effectiveness in the Arab-Israeli War of 1973.

By installing the antiaircraft missiles less than 24 hours after the helicopters' loss, the Syrians, in the view of analysts here, were signaling their abrogation of the reported tacit 1976 accord, which apparently did not bar use of helicopters, presumably because their introduction here had not been foreseen.

If Israelis do try to destroy the Bekaa Valley SAM sites, they are expected by specialists to use tactics similar to those they used in the 1973 Arab-Israeli war.

Then, Israeli planes first destroyed Lebanon's only military radar, which was passing on intelligence to the Syrians despite Lebanon's technically neutral status in that conflict. That radar station is reliably reported to be still inoperative.

Specialists say the Israeli Air Force could easily destroy the unprotected Syrian radar station clearly visible just south of Beirut International Airport.

With the Lebanese radar knocked out in the early stages of the 1973 conflict, Israeli jets attacked Damascus by flying in low over the Mediterranean, often late in the afternoon when the sun was in Arab gunners' eyes.

The planes skimmed over the Lebanon mountains, then flew low over the Bekaa Valley before flying over the mountain range marking the Syrian-Lebanese border and surprising Syrian air defenses. The aircraft then used the same route to return to the Mediterranean and to their home bases in Israel.

Theoretically, specialists said, Israeli planes now could surprise the SAMs in the Bekaa Valley and fire television-guided "smart" bombs from just over the Lebanese mountain range before scuttling back to the Mediterranean.

But any such maneuver would provide for no loiter time since the SA6s' computerized radar system could make any such delay fatal and the SA6s could destroy the attacking aircraft.

If diplomatic efforts succeed, deescalation might come, according to specialists, after a week or so during which the Israelis refrain from attempting to destroy the missile sites here. Then the missiles might be quietly removed and the Syrians resume helicopter resupply missions, but not the reported, but unconfirmed, use of helicopter gunships that Israel cited to justify its air actions Tuesday.

At least one of the downed Syrian helicopters was a Soviet-built transport helicopter, but some unsubstantiated reports said the other was a French-built Gazelle, which can be used as a gunship.