The House yesterday rejected a key weakening amendment to a major conservation bill that would set aside 100 million acres of Alaska as wilderness, parks, wildlife refuges and preserves.

By a 240-to-119 vote, the House rejected a move by Rep. Lloyd Meeds (D-Wash.) to cut in half the amount of acreage designated as wilderness in Alaska, which the Meeds amendment would have cut to 33 million acres.

To designate an area as wilderness, according to the Wilderness Act of 1964, is to make it "an area where the earth and its community of life are untrammeled by man." Those already living in wilderness areas could continue to live there, and hunting and fishing would not be affected, but almost all commercial activity, including road building, would be prohibited.

Supporters of the Meeds amendment argued that wilderness designations in southeast Alaska overlap two of the finest mineral prospects in the area. The area has a 16 percent unemployment rate, and wilderness designation could cost about 1,000 jobs in the timer industry, they said.

Opponents of the Meeds attempt said Alaska has the only remaining sizable amount of wilderness area in the country, and the bill offers at last chance to preserve it. The lands could be developed later if it is found the resources are vital, they argue, but if it was developed it would be impossible to restore it to wilderness. They also contend that 70 percent of the state's potential hardrock minerals and 95 percent of its potential oil and gas deposits are outside the designated aras and open to development.

Rep. Morris K. Udall (D-Ariz.), chairman of the Interior Committee, said the wilderness designations had already been cut in half fro what was originally intended."We have compromised, recompromised and compromised again," he said.

Earlier, the house had also defeated an amendment by Alaska's Republic Rep. Don Young that would have given the state the right to select 5 million more acres of land out of 16 million remaining to it under a land grant given Alaska by the federal government upon statehood in 1959.

Udall argued that the state would be given the full 104 million acres granted to it, but that it would not be allowed to choose the 5 million acres it wanted "in the middle of national parks. We shouldn't create inholdings in national parks."

Young argued they bordered the parks and weren't inholdigns and urged that the Congress "keep faith" with the Alaskans and give them the land they are entitled to.

The Alasaka lands bill, which would set aside an area as large as the state of California for parks, refuges, and preserves, is the largest conservation measure Congress has considered in this century. Though all the land in question is in the state of Alaska, in a single stroke it would double the size of the national park system.

Still it amounts to only one-third of the land in the union's largest state, a state twice the size of Texas.

The House, which is expected to easily pass the bill, will continue work on it today. However, its passage in the Senate is doubtful, since Alaska's two senators have threatened a fillibuster which Senate Majority Leader Robert C. Byrd (D-W. Va.) said he would not move to shut off. The Senate traditionally does not pass legislation affecting a state which the state's senators oppose. The Carter administration strongly supports the bill.