The House Interior Committee tentatively approved a bill yesterday to set aside a third of Alaska in parks, wildlife refuges and wilderness - which would double the size of the nation's park and refuge system.

The committee bill, while significantly less ambitious than the original version introduced by Rep. Morris K. Undall (D-Ariz.), is still considered by the Carter administration and environmentalists to be the most important conservation legislation of the century.

The debate over how to manage vast federal lands in the nation's largest state sparked intense lobbying on the Hill, with environmentalists fighting a coalition of oil and mining companies, building trade unions, the National Rifle Association and the state.

Udall said he was forced to accept compromises 'that kind of broke our hearts" because of "tremendous pressure" on committee members, especially from labor lobbyists and oil companies. "We were hanging on by one or two votes" on key provisions, he added.

Concessions include:

A reduction in overall park, refuge and scenic river acreage from 114 million to about 95 million acres, and in wilderness-disignated lands from 142 million to about 75 million acres.

A provision to allow mining and drilling in 67 million acres of park preserves and wildlife refuges in the event of a national need for minerals that other domestic sources could not meet.

The opening of a portion of the Arctic Wildlife Refuge, the nation's major caribou range, to government supervised oil drilling if an Interior Department study shows significant petroleum reserves.

The deletion of more than half of the proposed wilderness areas in southeast Alaska's national forests, including Misty Fjords, 2.5 million acres of spectacular coastal glaciers and snow-capped peaks. The area includes a molybdenum claim by the U.S. Borax Co.

The switch of several million acres from "parks" to "preserves," thus allowing hunting and possibly mining. For example, in the Wrangell Mountains, dall sheep, the section that would be made a park is relatively inaccessible snow-capped peaks, while the lowlands, where hiking is easier, would be a preserve, and thus open ot firearms.

"We've made tremendous concessions," said Rep. John F. Seiberling (D-Ohio) a sponsor of the Udall bill whose large, color photographs of Alaska dominated the crowded hearing room yesterday. "But we didn't give away anything we didn't have to in order to prevent the gutting of the bill."

The main challenge to the bill was led by Rep. Lloyd Meeds (D-Wash), who would have cut the wilderness to 33 million acreas - mainly in national parks. "The original bill was a no-growth, no-development, lock-up bill," he said, adding that land should be studied for oil and mineral potential before being designated as wilderness. About 700 timber-related jobs would be lost because of wilderness protection in national forests, he said.

Meeds may offer his substitute on the House floor in late April. The Senate Energy and Natural Resources Committee has yet to hold full hearings on the bill, but Meeds said its chairman, Sen. Henry M. jackson (D-Wash.), who would have cut the wilderness to 33 million acreas - mainly in national parks. "The original bill was a no-growth, no-development, lock-up bill," he said, adding that land should be studied for oil and mineral potential before being designated as wilderness. About 700 timber-related jobs would be lost because of wilderness protection in national forests, he said.

Meeds may offer his substitue on the House floor in late April. The Senate Energy and Natural Resources Committee has yet to hold full hearings on the bill, but Meeds said its chairman, Sen. Henry M. jackson (D-Wash.), "feels very much the way I do." The Senate is expected to further reduce wilderness areas - a move Seiberling says would cut into the heart of the bill.

"The most unique part of Alaska is to experience huge areas of wilderness without the impact of man," he said. "That was the experience of the pioneers. But one road or one power line across a wild landscape completely changes its effect on your mind."