MOSCOW, OCT. 7 -- Monkey business aboard an unmanned Soviet satellite has ground control here worried that a 12-day biospace research mission may have to end early.

The troublemaker is Yerosha, a lively young rhesus who, five days into the flight that began Sept. 29, managed to free his left paw from its restraining cuff and has been happily checking out everything in reach.

Yerosha's first gesture of independence apparently was to rip his nameplate off the electrode cap affixed to his head. Startled ground scientists first realized something was wrong when he turned up on the television monitor without it.

"What he did with it is still unknown," Tass said.

To find out what Yerosha might do next -- to himself, to a second monkey still attached to its restraints, to a collection of rats, fish and bugs along as fellow travelers and to a variety of instruments and scientific experiments aboard -- ground scientists have set up a Yerosha double. Another rhesus monkey is now sitting here below in a mock-up space capsule, left paw free and the same temptations within reach.

So far the other life forms aboard are behaving themselves, according to Tass, and everything is "in order" in other scientific compartments of the spacecraft.

An account of Yerosha's mischief -- his name, presciently given him before the mission, means "troublemaker" -- was reported last night by the Soviet news agency Tass and printed in today's Sovietskaya Rossiya.

The Tass story dwelt on the lighter side of the affair, but Yerosha's antics have people at the flight center of the Institute of Medical-Biological Problems worried that they may have to abort the mission -- embarrassingly as hundreds of scientists gather here from throughout the world for the Soviet Union's celebration of the 30th anniversary of its launching of sputnik.

At first, Tass said, both Yerosha and the other space monkey, Dryoma (his name means "drowsy"), were in top form, showing "brilliant mastery of the program of experiments." The two are fed only after they execute commands transmitted by lighted signals.

But since breaking loose, Yerosha -- apparently because he keeps moving around -- is not earning or getting all his food rations from the tube, Tass said, although there are no signs yet of a "hunger strike."

Yerosha was instantly pegged as the excitable one when his pulse rate shot up during lift-off to 200 a minute, and his temperature soared, Tass said. In contrast, Dryoma, "older and more reasonable," reacted normally to the pressure and weightlessness.

According to Tass, the concern is not only for the mission, which is carrying projects from a number of foreign scientists, including a crystal installed by U.S. experts, but also for Yerosha's health.

"Analysis of the entire situation, data received from the ground experiment and humanitarian considerations may at any time move the experts to decide on a descent," Tass said.The biosatellite is one of a series of Soviet tests of the capacity of living creatures to endure long periods in space. Last week a Soviet cosmonaut set a new record by spending 240 days in space.