A space walk planned for early Sunday morning was postponed until Monday because one of the astronauts aboard the space shuttle Columbia is suffering from motion sickness.

"There was no overall fear that we could not do it as easily on Monday as Sunday," director John Cox said late today at the Johnson Space Center here, where the flights of the shuttle are directed.

"It's just that we want to be conservative. Let's have everybody at 110 percent when we do something like this."

The sick astronaut, William Lenoir, vomited after breakfast this morning. Pilot Robert Overmyer also has been fighting nausea and threw up on Thursday, the first day in orbit. But he has apparently recovered from his bout with motion sickness.

The space walk is the last remaining major objective for the Columbia's crew, and Mission Control said it would take advantage of the flight's "flexibility" to increase the chances that Lenoir would be ready for the walk.

The decision was passed up to the shuttle crew at 5:25 p.m. (EST), shortly after they had retired for the day. Cmdr. Vance Brand, who took the radio call, replied simply: "We understand."

Flight directors told the crew to swap activities planned for Monday, when the astronauts had a light schedule of housekeeping and experiments, with those originally set for Sunday. The exact time for Monday's space walk was not announced by Mission Control.

Overmyer, who will supervise the space walk, has been responding to drug treatment and was feeling well enough today to eat a full breakfast.

But Lenoir, who is scheduled to make the space walk with astronaut Joseph Allen, has been having a rough time, and the last thing flight directors wanted was a sick astronaut vomiting inside his space suit.

"We talked privately to the astronauts three times today," Dr. Sam Pool, a flight surgeon here, told reporters.

Instructions were given for taking medicine, he said, and "after our third conversation, Lenoir reported he was feeling much better, was taking fluids but had eaten no solid food. This was just before supper."

Flight directors also do not want to make the space walk with one astronaut and have no plans to send Brand outside the shuttle just because he, like Allen, has not suffered from motion sickness.

"Bill Lenoir and Joe Allen are the two people trained to do this," flight director Tommy Holloway said. "Though it's technically possible [for Allen and Brand to space walk], we would not entertain doing that."

The original schedule called for the two astronauts to put on their pressurized suits and take a 3 1/2-hour walk, starting at 8 a.m. Sunday, up and down the spaceliner's 65-foot-long cargo bay.

Flight directors said privately they will continue watching how Lenoir responds to a combination of two drugs he took earlier today before the decision was made to delay the space walk.

Although Lenoir had been reported as feeling better, a televised broadcast from inside Columbia's cabin this afternoon showed that Lenoir was not moving around as well as the other three astronauts and was not smiling as much as the others.

Lenoir is taking a combination of Scopdex and Phenergan, described by officials here as the two most effective drugs for motion sickness.

Scopdex is a mix of scopolamine, which induces drowsiness, and Dexedrine, a well-known stimulant.

Tonight, flight surgeons said they had ordered Lenoir to take a sleeping pill in order to give him a chance for a good night's sleep.

Motion sickness isn't new to the shuttle program. It was a factor on the previous three flights, and the four astronauts who were sick on those flights all took Scopdex to battle the illness.

Phenergan, while not a new drug, has not been taken by a shuttle astronaut before; an astronaut has never taken it in combination with Scopdex before. Phenergan produces a more potent sedative effect than scopolamine, and astronauts have stayed away from the combination for fear of feeling too drowsy to get their work done or to enjoy the excitement of space flight.

Lenoir took the Phenergan as a suppository today in an attempt to make the space walk with Allen on time.

Though Overmyer was sick on Thursday and Lenoir began to feel nauseous on Friday, flight directors and surgeons covered up their illness until this morning, when Lenoir was sick enough to vomit and the possibility was raised that the space walk would have to be postponed.

Reporters here have asked daily since the mission began whether any crew member was sick. Each time, they were told that nobody had been ill and that the crew was feeling fine.

But after more than 20 years of sending men into space, the National Aeronautics and Space Administration is finding it has a real problem: half the astronauts it sends up will become space-sick. The condition seems worse on the shuttle than it was on Apollo moon flights, for example, because the crew has more room to move around.

Flight surgeons say it is almost impossible to predict which astronauts will get space-sick.

Earlier, Lenoir and Overmyer showed what one doctor called "superior resistance to motion sickness, no susceptibility." The doctor said that Brand showed the same kind of resistance and that Allen seemed the most susceptible of the four. Ironically, Allen and Brand are the two who have shown no ill effects from being in space.