Like an old Western movie, the new guys galloped in, pistols drawn, vowing to "change things around here."

These are heady days at the Interior Department. The new secretary, former Idaho Gov. Cecil D. Andrus, has sworn to manage the government's vast landholdings "in a manner that will make the three Rs - rape, ruin and run - a thing of the past."

Assisting him is a crowd of new young faces, many of them little known, but some of them former opponents of the industries that have traditionally swayed the department.

Andrus has taken on Western politicians - traditionally Interior's strongest allies - by vigorously supporting President Carter's move to kill what he calls wasteful water projects.

He has frightened oil companies by advocating more competition in the energy business and stricter offshore leasing policy. He has disappointed the coal industry with his support for the strip-mine bill. He has called on timber companies to stop logging around California's Redwood National Park.

Environmentalists are delighted. This Sierra Club dream-come-true represents a historic change for the sprawling 70,000-employee agency that manages roughly a third of all U.S. land, with vast oil, coal and mineral reserves.

"We have begun to make sweeping institutional and policy changes to end the domination of the department by mining, oil and other special interests," Andrus said.

"Our President . . . is canceling the blank check which once went to those who would exploit resources and pollute the environment in the name of progress.Business as usual has been put out of business."

And business, although Andrus says it will be an "equal partner," is worried.

"I imagine anybody whose role is to develop energy resources has to be apprehensive," said Carl Bagge, president of the National Coal Association. "I'm doing a lot of praying these days."

Charles DiBono, vice president of the American Petroleum Institute, says oil companies are looking at the department with some "nervousness . . . in a state of watchful anticipation."

An attorney with Standard Oil of Indiana, William Block, says the department has taken "a definite turnabout. The environmentalists are taking over. The people responsible for mineral development are being ignored."

Appointments are a clear sign of change. The key job of assistant secretary for energy and minerals is going not to someone with close industry ties, as has often been the case, but to Joan M. Davenport, 34, a government economist who was director of environmental assessment at the Federal Energy Administration.

"Every rock I lift up, I see another professional environmentalist," Bagge said. Environmentalists contend Andrus is simply ending the era of industry executives who join Interior to further business interests.

Two key jobs have been filled by businessmen who are little known to industries that deal with Interior: James A. Joseph, 42, a former vice president of the Cummins Engine Co., the first black to serve as under secretary; and Leo M. Krulitz, 39, vice president of the Irwin Management Co. of Columbus, Ind., and Andrus' former campaign manager, as solicitor.

Some Washington environmentalists have joined the department: The Audubon Society's Cynthia Wilson, 36, as Andrus' special assistant, and, from the Environmental Policy Center, Barbara Heller, 28, as deputy under secretary, and Joe B. Browder, 38, as an assistant in the Land and Water Resources Division.

Andrus says his appointments represent "a clear balance between West, North, South and East, between industry and environmentalists." Under Secretary Joseph adds, "We'll be in nobody's hip pocket."

But they are serving notice on the department's longtime constituents. "We intend and have begun to break up the little fiefdoms which have divided Interior for years," Andrus said.

"For too long each of the interests grazing, mining, timber and so forth - has had its own domain. The place was like a centipede, with each little pair of feet scuttling off in its own direction. "That is going to change."

For a start, the department is re-evaluating the costs and benefits of 11 water projects - including the Central Arizona Project and Garrison Diversion in North Dakota - all of which had the unquestioned support of the Ford administration.

"We are coming to the end of the dam-building era in America," says Andrus, in a statement that amounts to heresy in thirsty Western states. As Idaho governor, he experienced first-hand the safety and environmental problems of some Reclamation Bureau projects when the Teton Dam burst last year.

Andrus is cracking down on the offshore oil industry by refusing to renew 62 leases where oil and gas have been discovered, and production has been delayed. He also delayed a lease sale of Alaska pending further environmental studies.

In a February television interview, Andrus said he favors so-called "horizontal diverstiture" legislation to prohibit oil companies from developing competing sources of energy such as coal, nuclear and solar power. However he pleased industry by appealing a federal court decision halting off-shore leasing in the Atlantic.

Andrus said he wants to change the Mining Act of 1872, which gives industry relatively unrestriced access to public minerals. He favors some federally run exploratory offshore drilling - a domain industry jealouly guards.

Ranchers, another group that exercized considerable influence in the past, will see changes. "For too long, much of the land where the deer and the antelope play has been managed primarily for livestock, often to the detriment of wildlife," Andrus told the National Wildlife Federation last week.

"For instance, 'sheep-tight' fences have barred antelope from their traditional travel routes, causing needless mortality. Now we are beginning to require modification of those fences so that antelope can get through. This has not been popular with some ranchers . . ."

Interior Department agencies like the Bureau of Mines and the Reclamation Bureau, which have a history of dealing with Congress on their own, will be reined in, Andrus says. He is optimistic about his ability to win over bureaucrats who have advocated different positions in the past.

While Carter's proposed Energy Department would remove certain power marketing functions from Interior, Andrus says he intends to keep a hand in energy policy.

Andrus' pledge to increase Fish and Wildlife and Parks Service personnel and his call for a six-month moratorium on timber cutting near the Redwood National Park have endeared him to conservationists.

"He care about the things I care about," said Audubon's Cynthia Wilson, explaining why she joined his staff. "He cares about wilderness and wild rivers. Before, you'd talk to Interior [about an area] and they'd say, "That's all locked up.' Now they want to know what they can do to help."

At his confirmation hearing before the Senate, Andrus expressed this philosophy:

"If I'm faced with a decision of development with adequate safe-guards for the environment, I'll come down on the side of the development. If I'm faced with development without adequate safeguards, I'll come down on the side of environment."