The Fish and Wildlife Service yesterday lifted its six-year ban on the importation of kangaroo hides, a shift that upset environmentalists who say it will result in a resumed slaguther of the Australian animals.

"We are satisfied that the kangaroo is adequately protected in Australia and that a carefully monitored trade can be allowed safely at this time," said Dr. Eugene Hester, acting deputy director of the service. The Interior Department will review the decision in two years, he said.

Hides from the large Eastern Gray, Red and Western Gray kangaroo varieties were imported by the millions until banned in December 1974, when the marsupial was labeled a threatened species. The U.S. market was the world's largest for the hides, which were processed here and made into athletic shoes, handbags, coats and novelty items prized for their strength and softness.

Lewis Regenstein, vice president of The Fund for Animals, which sought the initial ban, charged yesterday that the policy shift is "devastating. It means millions of kangaroos will be killed for the U.S. market." He said the decision foreshadows others expected to remove the African leopard from the endangered species list next month.

The killing techniques for kangaroos resemble those for baby seals. Hunters work at night with searchlights, clubbing or shooting adult kangaroos in order to disable but not kill them until they can be picked up over the next few days. This is a technique to keep the meat fresh for use as pet food, Regenstein said. Babies, called joeys, are decapitated or clubbed to death on the spot.

The Australian government reported last year that 3.5 million kangaroos were killed, half of them illegally, for the European market, each skin bringing about $4. Hester said the current adult kangaroo population in Australia, which includes many small species, is about 32 million, and that the exporting states had set up population surveys, license systems, quotas and monitoring that would guarantee no further abuses.