Israeli fighters shot down a Syrian Mig21 interceptor today in the first aerial combat over Lebanon in 11 months, the military command announced.

The clash, in which Israelis said they lost no planes of their own, followed by five days the largest Israeli attack on southern Lebanon since the invasion of March 1978. It was interpreted by Israeli military sources as a sign of Syria's determination to show its Palestinian followers that Israeli attacks into Lebanon will meet at least some resistance.

That policy was Syria's primary motivation in previous engagements with Israeli jets over southern Lebanon. Those included an encounter in June 1979 in which five Syrian Mig21s were shot down, and a second fight last September in which five more of the Soviet-made Syrian jets were knocked out by Israeli fighters. Israel lost no aircraft in either of those clashes.

The military command said today's combat erupted over southern Lebanon when an Israeli reconnaissance patrol was challenged by the Syrian intercepotrs. A "brief dogfight" ensued, a communique said, during which the Syrian jet was shot down. All the Israeli planes returned safely to their bases, it added.

An announcement in Damascus acknowledged the loss of one Syrian fighter, whose pilot was said to have parachuted to safety, and declared the Syrian jets also downed one of the Israeli warplanes. Reporters from Beirut said the Israelis appeared to be flying in to attack Palestinian positions, not simply making a reconnaissance run.

The Israeli reference to a dogfight implied that the fight was not the result of an Israeli trap, as was the last encounter Sept. 24 when high-flying Israeli F15 jets blasted the Syrian Migs with radar-guided missiles before the Syrian pilots had even sighted them.

The dogfight today, which took place about nine miles east of Sidon, involved four Syrian Mig21s and an unspecified number of Israeli Kfir jet fighters and other aircraft.

Israeli jets regularly fly reconnaissance missons over Palestinian and Syrian positions in Lebanon, frequently setting off Palestinian antiaircraft batteries in mostly futile barrages. Only occasionally do Syrian planes fly in to challenge the Israelis, despite the presence of more than 20,000 Syrian troops in Lebanon and despite a Syrian policy that includes Lebanon in its strategic air space.

The Syrians are forced to use that strategy because the Israeli F4 Phantoms Skyhawks and Kfirs -- backed by the sophisticated F15s flying high protective cover -- are more than a match for the Syrian Mig21s. Syrian President Hafez Assad has been reluctant to dispatch his more sophisticated aircraft into the skies over Lebanon, even though the high-flying Soviet Mig25 Foxbat bomber-interceptor and a dozen Mig27 fighters generally considered able to compete effectively with the F15.

Diplomats in Damascus attribute his hesitation to a lack of training for Syrian pilots on the sophisticated new Soviet fighters and Assad's traditional prudence, which leads him to husband his forces for more direct threats to Syria than the Israli attacks in southern Lebanon.

In any case, they say, Assad's purpose is served even if his Mig21s are shot down. On one hand, he is able to demonstrate to the Palestine Liberation Organization that the Israelis can not attack PLO forces in Lebanon without a challenge. On the other, the risk of aerial combat spilling into a broader conflict raises the diplomatic temperature in the region and raises appeals to Isreala to be moderate in its war against the guerrillas encamped in southern Lebanon.

The air combat coincided with two explosions around Jerusalem that the PLO in Beirut and Damascus said were part of its continuing war against the Jewish state.

One person -- an Israeli gas station attendant -- was reported killed in a blast from a bomb planted in a trash can near pumps at a station about 10 miles west of Jerusalem on the old road to Tel Aviv, police said. One other person was critically wounded in a fire ignited by the blast, and 10 others, including some tourists, were slightly injured, a communique said.

An earlier blast at a hitchhiking station for Israeli soldiers on the out-skirts of Jerusalem caused no injuries and only slight damage, the Israeli Army said.

In another development reported by news agencies, an explosion in a bobby-trapped car killed five Lebanese and injured more than 30 today in Rifon, 20 miles north of Beirut, police said. The blast occurred as patrons were leaving a movie theater in the main square of the sumer resort town in an area controlled by Lebanese Christians.

The explosion left a nine-foot-wide crater in the road. No group claimed responsibility.