President Reagan's nominee for the post of assistant Interior secretary in charge of energy and minerals is being viewed within the agency as signaling a green light for Secretary James G. Watt's pro-development agenda.

William P. (Perry) Pendley, formerly the No. 2 official in the energy and minerals shop, is described by a high agency official as "Jim's ideological brother."

Pendley, 38, a former Republican aide on mining issues for the House Interior Committee, is one of the agency's strongest proponents of expanding the leasing of coal, oil, gas and the other natural resources that lie under public lands.

He helped draft the administration's policy for speeding the development of strategic minerals, which called for revoking restrictions on large stretches of western land. He also was an early advocate of looser restrictions on mineral exploration in wilderness areas. And he has pressed for lowering the floor price for leases on government-owned coal, oil and gas, arguing that it is not up to the government to interfere in the free market.

As assistant secretary, Pendley would oversee the leasing of offshore waters for oil and gas exploration, certain wilderness development questions, strip-mining regulation, the U.S. Geological Survey and a wide range of mining issues.

The Senate Energy and Natural Resources Committee has yet to schedule confirmation hearings, which some say may become a forum for examining the Watt record. There is speculation that Reagan will appoint Pendley during a congressional recess, allowing him to assume the post without confirmation.

For another vacancy, Watt is expected to name David Russell, another of his strong philosophical compatriots, to head the Minerals Management Service.

GRIZZLY DIPLOMACY . . . Out in the wilds of Yellowstone National Park, one of the more celebrated schisms in wildlife research appears to be narrowing.

For about a decade, there has been a stony silence between two groups fighting to save the imperiled grizzly bear: National Park Service officials and brothers John and Frank Craighead, leading biologists in the grizzly field.

Now, with the government engaged in a major push to save the grizzly and with the number of females around Yellowstone estimated to be as low as 30, the two sides have recently begun a rapprochement that appears to have excited wildlife biologists. Yellowstone Superintendent Bob Barbee met last month with John Craighead--the first such meeting between the brothers and a Yellowstone superintendent in years.

The Craigheads are now serving on a task force studying whether to try "supplemental feeding" of the bears, that is, placing elk carcasses around the backwoods to manipulate their movements rather than pursuing a strategy of non-intervention.

That strategy started 14 years ago and helped precipitate the split. At the time, Yellowstone abruptly closed its open garbage dumps, which had become prime feeding spots for the bears. The park's theory was that grizzlies had become "hooked" on human garbage, and should return to the wild.

The Craigheads warned that the sudden shutdown would force bears to prowl campgrounds in search of food, where campers would kill them in self-defense. In fact, grizzly shootings soared after the dumps were closed, but Park Service officials still contend that the closings were not the reason. As the feud escalated, each side accused the other of arrogance, and communication stopped.

Now the channels are open, and Barbee says the bears' situation is too grim to allow them to close again. The grizzly population in the Yellowstone area, once one of the bear's prime stomping grounds, is now estimated at 200 and falling.

PAYING TO PLAY . . . Interior officials are considering a plan to charge user fees for all sorts of free activities in national parks, and to plow the money into Watt's program to rebuild park roads and other aging facilities. Among the freebies most likely to be targeted are weddings, cross-country skiing and the filming of commercials and movies, officials said.

But it would take legislation to use these fees for park maintenance (they now go into the general Treasury), and Watt was unsuccessful last year when he asked Congress to divert entrance fees into the fix-up program. By law, they are supposed to be used for buying new park land.

Even if the pay-as-you-play scheme flies, some things will still be sacred. Asked if visitors could be asked to pay a quarter to watch Old Faithful erupt, a spokesman gasped and answered: "No. God, no."