With the Interior Department's blessing, the nation's seventh-largest oil company is negotiating to build an air base, with runways the length of Washington National Airport's, on a pristine Alaska island that Congress voted in 1970 to keep "untrammeled by man."

Alaska's St. Matthew Island is "as close to pure wilderness as can be found in the United States today," a recent Interior report says. The treeless stretch of tundra and cliffs in the Bering Sea is home to arctic foxes, sea lions, gray whales, several million sea birds and no humans.

Once part of the land bridge between Asia and North America, it harbors species from both continents. To keep it that way, Congress placed the island in the wilderness system in 1970, a blanket ban on development.

When Atlantic Richfield Co. (ARCO) asked Interior a year ago for permission to build an airfield and harbor on St. Matthew for offshore oil and gas exploration, the answer was a flat no. The Wilderness Act spoke clearly: "There shall be no commercial enterprise . . . no landing of aircraft" in wilderness areas.

But that was before Interior's U.S. Geological Survey directed the oil company in a 1981 letter to explore "appropriate administrative channels." ARCO took the advice.

The result: ARCO struck a bargain with a group of Alaska Indians, Aleuts and Eskimos to run interference through the wilderness laws.

The plan is for the natives to acquire the island land, invoking a federal law designed to protect native claims, and then lease it to ARCO at a handsome profit.

One year after the initial request, Interior has deployed more than a dozen biologists, real estate specialists, marine refuge staffers, offshore oil exploration analysts, researchers, attorneys and ecologists to study the air base proposal, and has produced a solicitor's opinion that, in the words of Interior's deputy undersecretary for Alaska, "gives it the green light."

All parties appear pleased with the bargain. ARCO would get its air base; the natives would make millions of dollars, although no figure has been set yet for the lease. And Interior says the nation's wildlife system would be a winner, too, because the natives will be required to trade land within existing wildlife refuges for the 2,560 St. Matthew acres.

"We expect to get back a lot more land than we give up," said Interior Undersecretary William Horn.

But on Capitol Hill there are cries that the proposed deal circumvents the will of Congress. In Alaska native groups are questioning the idea of making money at the expense of the state's natural heritage.

Environmentalists are threatening to sue in the name of the Wilderness Act and the 1980 Alaska Lands Act, which reaffirmed St. Matthew's wilderness status. And Interior is caught in a balancing act between its conflicting missions of developing offshore oil and protecting the environment.

Interior's Fish and Wildlife Service in Anchorage is still negotiating the land exchange with Cook Inlet Region Inc., the corporation of Indians, Aleuts and Eskimos involved in the swap. The only snag is how much land the natives will cede in return for the St. Matthew acreage.

"Basically, they want more land out of our clients' hides," said an attorney for the natives.

If the Anchorage office approves the exchange, it will be forwarded to Washington for approval, eventually to Secretary James G. Watt.

Horn stressed that Interior will block the deal unless the natives trade enough land to constitute a "net benefit" to the wildlife system. Interior has not yet calculated the potential wildlife loss from development on St. Matthew, but that will eventually be considered, he said.

A "net benefit" swap, Horn said, would meet the spirit of the Alaska Lands Act, which was intended to preserve the state's cultural and natural heritage.

Rep. John F. Seiberling (D-Ohio) disagrees. "Congress made it wilderness, and it's up to Congress to take it out of wilderness," said Seiberling, an author of the Alaska Lands measure. He said he might vote for such a measure if Interior submitted it to Congress and proved it was needed, but "I do not like the precedent of allowing the executive branch to make this kind of an inroad in a wilderness area."

The Wilderness Society, the Audubon Society and the Sierra Club have promised to sue to try to block the swap if Interior approves it.

The groups have obtained an internal memo from the Fish and Wildlife Service saying, "Development on St. Matthew is completely contrary to the wilderness values of the refuge. Construction on even a small section of the island and the commencement of air traffic would degrade the wilderness value of the entire refuge." Air traffic, the memo says, could kill substantial numbers of seabirds, causing adults to abandon chicks and eggs.

The legal debate centers on a section of the Alaska Lands law that allows Indian corporations to exchange lands within wildlife refuges for other public lands. Seiberling, who helped write the section, said it was designed to help Interior eliminate private holdings within wildlife ranges, not to erase one private holding and create another.

But Horn, a former Republican staff member on the House Interior Committee, contends that the conservationists and Seiberling are misreading the law. So does Sen. Ted Stevens (R-Alaska), who in a 1970 speech on the Senate floor supported the designation of St. Matthew as wilderness "to ensure that the natural habitats and breeding grounds of the special seabirds and wildlife on these islands remain well protected."

"We wrote a little of that law too," said a Stevens aide. "Contrary to Mr. Seiberling's views, that law was written to allow exactly what's being done. That base would help protect human life during oil exploration. We feel it's in the interest of the state and the country."

That is essentially the case ARCO made in a June letter to Assistant Interior Secretary G. Ray Arnett, who oversees fish, wildlife and parks programs. Arnett, a former ARCO geologist, has left the matter to his subordinates, according to a spokesman.

ARCO wants to use the proposed St. Matthew base for an exploratory well the company plans to drill next summer, in partnership with 17 other oil companies, according to a company spokesman. The oil firms will use findings from the test well to prepare bids for a 1984 Interior Department lease of oil and gas resources in the Navarin Basin.

The basin is so isolated from existing land bases that it would take more than two days to rescue crewmen or clean up spills in case of emergencies, ARCO Vice President G.T. Wilkinson wrote Arnett last year. Rescue crews could reach the well site from St. Matthew in less than half a day, Wilkinson said.

ARCO's plans for the base include two 6,500-foot runways (National's longest runway is 6,870; its shortest is 5,212), a 1 1/2-mile gravel roadway, warehouses and a harbor. There would be regular takeoffs and landings of large supply planes, a spokesman said. The base would remain active for nine to 50 years, depending on whether oil is discovered, Interior officials said.

ARCO took its problem to Cook Inlet, the fastest growing of Alaska's regional native corporations, because the oil company already had lease arrangements with the group on the mainland, said Cook Inlet Executive Vice President George Kriste.

All the attention has prompted intense soul-searching within Cook Inlet, Kriste said. One of 13 native enterprises created by the Alaska Native Claims Settlement Act, the group is charged with making profits for its native shareholders and also with preserving their cultural heritage--a twin mission with inherent conflicts, particularly as oil money floods into Alaska, Kriste noted.

"These groups were founded on the great premise: Go out and make a buck, guys, and at the same time, hold onto whatever culture you can," Kriste said. "That gives them the highest order of social responsibility. But if you look at the history of American business, most corporations didn't become socially responsible until they became fantastically wealthy. It's quite a big order to try to preserve your culture and at the same time bring your shareholders into the 20th century."

One reality of the modern world is litigation, Kriste said, noting that if the environmentalists bring suit, it will be against the Indians, not ARCO, because the oil company is on the sidelines at this point.

"We debated this thoroughly," Kriste said. "We're not doing this as a front for oil companies. We're doing it for economic value to ourselves."