Two agencies that administer national forests and other federal lands seek to swap jurisdiction over 35 million acres in what they describe as an efficiency move, it was announced yesterday.

But environmentalists challenged the proposal as an attempt to weaken protection for national forest lands.

The move would consolidate adjoining blocks of forest managed by the Agriculture Department's U.S. Forest Service and parcels controlled by Interior's Bureau of Land Management.

Most of the acreage involved is in 11 western states, but tracts throughout the east also are involved, said Forest Service Chief R. Max Peterson.

Many people, he said, "have never understood why there should be a different set of rules for land on one side of an imaginary federal boundary and another set for federal land on the other side," he said at a news conference.

However, Charles Clusen of the Wilderness Society said, "We fear that today's announcement to swap more than 30 million acres of western public lands is one more step in the Reagan administration's continuing campaign to increase the commercial uses of federal lands to the detriment of the environment and the public."

BLM Director Robert Burford said the proposed land swap was prompted by a recognition that "many of our historical jurisdictions are illogical and inefficient."

Under the proposal, 19.5 million acres run by the BLM would be turned over to the Forest Service in exchange for 15.5 million acres under Forest Service management.

The agencies did not specify areas to be exchanged, saying those details will be included in legislation to be sent to Congress in May.

The administration estimates annual savings of $25 million to $35 million once the swap is complete, Agriculture Secretary John R. Block said.

The swap would not alter receipt-sharing programs between the agencies and local governments for mining, grazing and other land uses, Peterson said.