IN EASTERN SAUDI ARABIA, JAN. 18 (FRIDAY) -- The roar of a Patriot missile rocketing toward an Iraqi-launched Scud shook a large air base in this region about 4:45 a.m. local time today, and a few moments later the incoming ground-to-ground missile was destroyed over the Saudi desert.

The Patriot's success in shooting down the Iraqi missile was an important demonstration of U.S. military technology. The Patriot had never before been tested in actual combat, military sources here said.

About eight minutes before the Patriot was launched, massive numbers of aircraft took off from the base simultaneously, apparently in an effort to avoid being hit by incoming missiles. The take-off was thunderous, rattling nearby buildings.

Following the dispersal of the U.S. and British warplanes, Saudi civil defense officials sounded air-raid sirens, imposed the highest level of chemical-weapons alert, and ordered civilians into shelters.

For the next hour and 25 minutes, Saudi civil defense officials and U.S. military officials at the base were in apparent conflict over the likelihood that additional missiles were on the way and whether chemical-weapons attack was likely.

The joint Saudi-U.S. military base remained at a high level of readiness even after Saudi civil defense officials declared the emergency over. Military officials had worried that because Iraq seldom fires a single Scud, the missile intercepted by the Patriot could have been an effort by Iraqi forces to test their aim for a heavier attack on the base.

The Patriot missile system recently has been sent to Israel but has not been fully deployed there.

Patriot missiles have been installed at this base and other strategic military facilities throughout the Arabian Peninsula. The Patriot is a highly computerized high- and medium-altitude U.S. Army air-defense system.

The unit deployed here is normally based in Fort Bliss, Tex.