LOS ANGELES (AP) _ A Pegasus rocket was launched from a B-52 bomber Wednesday, but the Pentagon lost contact with it and didn't know if it managed to place seven tiny military communications satellites in orbit.

''An anomaly may have occurred'' as the four-stage rocket's first and second stages separated, said Army Maj. Robert Bonometti, mission director for the Defense Advanced Research Projects Agency (DARPA).

He said it could be hours before officials were able to restore contact with the Microsat satellites to determine if they reached their near-polar orbits 447 miles above Earth.

The Defense Department and Orbital Sciences Corp., which helped build Pegasus, announced the problem nearly three hours after the rocket was launched. If the satellites didn't reach orbit, it would be the second failure of an Orbital Sciences rocket in a month.

The $18 million experimental mission by DARPA was the second launch of a Pegasus rocket. It was supposed to reflect a trend toward use of numerous, small, inexpensive satellites, which some hoped would be more useful for battle commanders and more reliable than single, large satellites.

The NASA B-52 bomber took off from Edwards Air Force Base at 9:25 a.m., carrying the 50-foot-long Pegasus under one wing.

The four-stage rocket - with a 22-foot-wide delta-shaped wingspan - was launched from the plane at 10:33 a.m., when it was 43,100 feet over the Pacific about 60 miles southwest of Monterey, said Nancy Lovato, a spokeswoman for the National Aeronautics and Space Administration.

Bonometti said the rocket's three solid-fuel stages ignited successfully, despite the possible separation problem, and a liquid-fuel fourth stage ignited as planned.

But bad weather prevented a tracking plane near Antarctica from learning if the fourth stage fired again as planned, and a ground station in the Indian Ocean region never received word from the rocket or satellites as expected, Bonometti said. Officials believe the rocket's guidance system could have corrected for the separation problem, he added.

Each Microsat weighs only 49 pounds, is 7 1/2 inches high and 19 inches wide.

The experimental Pegasus and its payload weighed 41,000 pounds, said Laura Ayres, spokeswoman for Virginia-based Orbital Sciences Corp., which developed Pegasus in a private joint venture with Hercules Aerospace Co. of Delaware.

DARPA, the Pentagon's research arm, contracted for three Pegasus launches. The first was in April 1990, when a Pegasus orbited a small Navy communications satellite and conducted environmental and magnetic field studies. Ayres said another Pegasus is to orbit an Air Force science satellite this fall.

The Microsats were meant to demonstrate the usefulness of small satellites for quick communications among battlefield and naval commanders, and will be used during the next three years in military exercises, Bonometti said.

Another goal is to develop miniature technology so big sophisticated satellites can be reduced in size and cost, said Air Force Col. Edward D. Nicastri, DARPA's assistant director for space systems.

The Microsats can provide quick voice, data, fax and picture communications among commanders in the 1,500-mile ''footprint'' of a single satellite, and also can pick up ''E-mail' messages from one part of the Earth for later delivery when the satellite passes over another point.

Orbital Sciences hoped Wednesday's launch would boost its fortunes. Last month, the company's 50-foot Prospector rocket carrying scientific experiments careened out of control and was destroyed shortly after launch from Cape Canaveral, Fla.