After months of training, Columbia's astronauts successfully completed the most difficult medical procedure ever attempted on someone in space - guiding a tiny needle into a leg nerve.

The nerve, located behind the knee, is impossible to see and nearly impossible to feel.Yet the shuttle's medical men managed to find the nerve in one another within the allotted 40 minutes, said Dr. David Robertson, a Vanderbilt University blood pressure specialist who trained the crew. In one instance, they found it within 10 minutes.

"We think that's just great," Robertson said Wednesday. "This is a very, very difficult thing to do on Earth, and the idea that it can be done in space is no doubt astounding to many people.

"This is the most difficult human experiment that has ever been attempted in space," he said.

Robertson's experiment is aimed at understanding and, hopefully, easing the blood pressure problem and accompanying dizziness suffered by many astronauts when they return to Earth. An estimated 500,000 Americans experience the same thing every day when they stand up.

"We see the astronauts as a very good model for this," Robertson said.

Two of the crewmen had the needle temporarily inserted in the nerve on Wednesday. Two others had it done Tuesday. No one complained of pain.

The astronauts found the nerve by listening to electrical nerve activity, which was picked up by the needle and amplified. The signals traveling from the brain to that nerve then were monitored as the men floated in a chest-high depressurization chamber and stuck their hands in an ice-cold mitten.

Columbia's crew also performed surgery Wednesday - on baby rats.

The two doctors, Jay Buckey Jr. and Dave Williams, cut into the legs of six anesthetized baby rats as part of a study of how nerves develop in weightlessness.

It was the first surgery ever performed in space on creatures meant to survive. Previously, astronauts merely dissected dead animals.

The doctors would have operated on one or two more rats if not for the surprisingly high mortality rate among the rodents aboard the shuttle.

The death count rose Wednesday to 52.