Astronaut Kathryn D. Sullivan today became the first American woman to walk in space and for 3 1/2 hours performed tasks requiring a kind of patience, dexterity, sta- mina and strength that for more than 20 years were believed in the space program to be unique to men.

Sullivan, an oceanographer and geologist, and astronaut David C. Leestma stepped into space at 11:44 a.m. EDT and returned to Challenger's cabin just before 3 p.m. EDT. Orbiting 138 miles above Earth at 17,500 miles an hour and each held down by a single tether, they worked in daylight and darkness to rehearse a critical fuel transfer, stow a troublesome antenna and photograph their efforts for ground engineers.

Sullivan kept pace with Leestma all the way as they moved up and down the 65-foot-long cargo bay in the difficult and tiring weightless environment.

"Look at the ground," Leestma told Sullivan when they took a break. "We're over a beautiful part of Canada, Cape Cod is beautiful and we're coming up on New York."

"There are a lot of Sullivans down there," replied his New Jersey-born partner.

With most of their mission accomplished and with Hurricane Josephine well out of the area and on its way toward Cape Hatteras, Challenger and its seven-member crew are still due to land as planned at Florida's Kennedy Space Center at 12:25 p.m. EDT Saturday.

Sullivan, 33, may be not only the first American woman to walk in space but perhaps the last, at least for awhile. She is the only one of the eight women astronauts who has been fitted for a spacesuit. At 5 feet 6 and 150 pounds, she is also the most robust of the eight.

For years, the men who run the astronaut program at Houston's Johnson Space Center have felt that women lack the upper-body strength needed for spacewalking and performing heavy work in the weightlessness of space while wearing a bulky spacesuit and holding handrails and foot restraints for balance.

Almost all the work done by spacewalkers involves the arms, shoulders, neck and hands. Using any kind of tool requires upper-arm strength because there is no force of gravity helping along. The earliest spacewalkers had great difficulty in learning to cope with weightlessness and the first few returned to their spacecraft cabins exhausted and almost dehydrated.

Before today, there had been 38 spacewalks by Americans, all men. Last July Soviet cosmonaut Svetlana Savitskaya became the first woman spacewalker.

The main task for today's spacewalkers was to connect a fuel line between a tank holding 210 pounds of toxic hydrazine fuel and an empty second tank. The exercise was a rehearsal for a mission when shuttle astronauts will attempt to refuel a Landsat satellite that has been useless since running out of fuel for aiming its cameras and instruments at Earth.

"This is not like driving up to your gas station and getting the hose and sticking it in," Flight Director John Cox explained. "There are more pieces to this operation than you can imagine."

After backing into space from Challenger's cabin, Leestma and Sullivan made their way to the rear of the cargo bay where the two fuel tanks and a tool box were positioned. Leestma fastened his feet to a pair of restraints in front of the fuel tanks and Sullivan held a handrail beside him.

For the next two hours, Sullivan handed Leestma wire cutters, a spanner and tools to remove dust covers, seals and nuts. She also photographed Leestma at work and held a flashlight for him during the period when Challenger was in darkness.

The job was anything but easy. Once, Leestma snapped: "I can't get this damned thing to work." Another time, he couldn't locate a valve as the spacecraft moved into darkness.

"It's really eerie out here," Leestma said, and laughed.

Echoed Sullivan, "It's hard to believe these things"--juggling a flashlight, a tool and holding onto the handrail at the same time.

Finally, the two astronauts finished the job. "That's great, that's great," Sullivan said as Leestma gave a last twist on a tool that removed a seal and opened the fuel line.

The physical transfer of the fuel was not scheduled to occur until late tonight or early Friday, when one of the crew members would throw a switch from inside the cabin.

The spacewalkers' last job before going back inside was to move a wobbling radio dish antenna so that the cargo bay doors do not bang into it when they are closed on Saturday before Challenger returns to Earth.

Leestma and Sullivan took less than 30 minutes to move the antenna and lock it in place.

The spacewalk was not without its humor or incident.

As the astronauts reentered the airlock, Leestma collided with the hatch, knocking loose a valve cap that floated behind him toward the rear of the cargo bay. "No, no, no," Sullivan exclaimed. Leestma muttered an expletive.

"Better luck next time," Crippen said from the cabin, "but can you go back and get it?"

Leestma moved back and retrieved the cap. Almost as if she hated to end the spacewalk, Sullivan went back out to help. "Here we are again at the hatch," Leestma said. "You want to stay out here an extra minute?" Replied Sullivan: "Twist my arm.