A space-sick squirrel monkey began eating solid food for the first time in orbit today and a second broken instrument in Spacelab was repaired, giving the space shuttle Challenger's crew members a welcome lift as they went into the final days of their science-crammed mission.

"The eating crisis is over," said Dr. William E. Thornton, one of two doctors handling two monkeys and two dozen rats riding on Spacelab with its seven-man crew.

"I couldn't feed him fast enough and he ate everything I gave him with both hands. I would not have believed the effect a caring human hand could have on an animal."

Earlier in the day, Thornton worried that the monkey might be so sick that it wouldn't eat any solid food during the mission. He asked and received permission from flight directors at the Johnson Space Center to hand-feed the monkey.

Thornton reached into the cage with banana pellets in both hands, and the the round-faced monkey began grabbing and devouring the food.

"He's taking everything he can and now he's over pushing the automatic feeder," Thornton said. "It's amazing. I think I'm going to leave him on automatic."

For the second consecutive day, a Spacelab crewman was able to repair a damaged instrument and restore it to service inside the $1 billion laboratory. First, physicist Taylor W. Wang suffered through three days of patiently testing his instrument's power supplies until he found and bypassed a short-circuit in one of three electronic boxes. Today, physicist Don Lind performed a similar procedure to restore service to a cosmic ray counter.

"What he did was to swap connector cables that hooked the experiment up to Spacelab's computer," Flight Director Gary E. Coen said. "There was a little bit of luck involved, since Lind was able to use a connector cable on another experiment that failed yesterday, but the point of it all is our in-flight maintenance experts prevailed once again."

The 24 rats aboard Spacelab were feeling fine, just as they had when they left Earth last Monday. The first television broadcasts of the rat cages were transmitted to Earth today, showing several rats tumbling like gymnasts with tails, doing somersaults and cartwheels the entire time the cameras observed them.

While they were able to deal with animal waste a lot better today than they had earlier, the seven crewmen still were griping about the waste that had poured out of the cages anytime the animals' food trays or waste trays were being changed.

"Everything comes forward out of the lab and we see it up here in the cockpit in 20 or 30 minutes," Challenger pilot Frederick D. Gregory complained. "It wasn't food that came out. That's our problem."

Challenger Commander Robert F. Overmyer said he could tell when the animal cages were being changed by the crowd that gathered near the tunnel connecting Challenger's cockpit with the 23-foot-long Spacelab in the shuttle's cargo bay.

He said it was like feeding time at the zoo -- with astronauts running around wearing surgical masks and holding vaccuum cleaners, sucking up the mess before it reached the shuttle cockpit.

"Unbelievable," he said. "People are banging into each other every which way, trying to move around in here."

Meanwhile, Gregory, 44, a Washington native and the first black to handle the controls of the shuttle, appeared briefly on television in Challenger's cockpit for the first time.

Gregory beamed at the camera and described to the world how he felt, what he was doing and what he was seeing as he circled Earth every 90 minutes.

Gregory wore a T-shirt marking the 20th reunion of his 1964 Air Force Academy graduating class.

"I've got a lot of classmates down there watching me," Gregory said to Astronaut Michael Mullane in the Mission Control Center this morning. "And they all expect to see the silver and blue Air Force Academy colors displayed prominently in the cockpit."

"That's an obnoxious T-shirt," replied Mullane, a West Point graduate.

"We're having a great time up here," Gregory continued. "The whole crew feels absolutely great. We've seen so much of the world that there's not much we can do up here but look out the window all the time.

"We just came up over a very cold-looking country the Soviet Union where there was a lot of ice and snow. We hope to see Hawaii shortly, but I don't know if we'll be able to get any pictures. It's still pretty cloudy there."

Gregory managed to join the fix-it team aboard Spacelab today when he used cardboard strips and some wire to repair a damaged movie camera.

"We're the crew that launches with broken equipment and brings back good equipment," he said with pride.