For the second time in three days, two spacewalking astronauts moved into the cargo bay of the space shuttle Atlantis and worked flawlessly through construction procedures that may be used to erect space stations in Earth orbit.

Changing the way they worked during their first spacewalk, astronauts Jerry L. Ross and Sherwood C. Spring last night rebuilt the 45-foot-high aluminum tower they erected Friday night.

This time Ross rode on the end of the shuttle's robot arm to snap together the last section of the 200-pound tower.

Later, the two astronauts attached an American flag to the tower and Spring rode the arm while moving the tower around effortlessly with his gloved hands.

"Let's go build a space station," Ross shouted. Spring said, "I bet we make it in a few years."

Ross and Spring took turns planting their feet in restraints at the end of the shuttle's 50-foot-long mechanical arm, which was being run from Atlantis' cockpit by astronaut Mary L. Cleave, the only woman in the seven-member crew. Cleave extended the arm almost as far as it would go so Ross could place the last of 10 aluminum structures on top of the four-story tower.

"What a weird sensation," Ross said as he hung at the end of the arm, more than 40 feet above the open cargo bay.

"I can't see anything but the struts of the top bay in front of me, and I have no idea where the arm is taking me," he added.

Cleave replied, "I always know where I'm taking you, Jerry."

While riding the arm, the astronauts moved the tower in different directions, as if space construction were child's play.

"It feels very easy," Ross said. "I can go anywhere I want with it, and I only need to use a little bit of finger pressure to move it around." Once, Ross pointed the tower at the full moon as Atlantis raced along at 17,500 mph 230 miles above the Earth.

"I can bring it down so I've got the moon right in the middle of a triangle," Ross said. "Bingo! Smack dab in the middle."

When it came Spring's turn to ride the arm and move the tower, he rushed at the job a little too fast and came close to hitting the inside of the cargo bay with one end of the tower.

"That's too close, Woody," Ross said to Spring. "Slow down."

So eager were the two spacewalkers to get outside the shuttle cockpit yesterday that they were in suits and ready to work almost an hour ahead of schedule.

They left the shuttle airlock and entered the cargo bay at 3:25 p.m., a half hour earlier than planned.

"We've got a couple of bit-chompers up here," Atlantis Commander Brewster H. Shaw Jr. told the Mission Control Center in Houston. "What do you say we get this thing started."

Ross and Spring appeared to slow down a little from their hectic Friday night pace when they did all the construction tasks they were assigned in a little more than four hours.

They did a little more sightseeing this time, watching lightning storms over central Africa and describing the clouds that covered most of the United States.

"It's clear over California though," Ross said. "I can see 29 Palms and Catalina Island in one direction and a lot of Baja and Mexico in the other."

When they rebuilt the tower, the two astronauts took turns stringing cable along the four-story metal frame the way future crews might lay cable for a space station. They also removed parts of the tower and replaced them with other parts, pretending to be space repairmen. While at the end of the arm, Ross and Spring called out instructions to Cleave on where they wanted to be moved.

"Thanks for the ride, Mary," Spring joked. "Can I have another ride later? I promise I'll be good."