Denver — What do you do when your nemesis begins acting friendly?
That is the situation the US environmental community finds itself in with Interior Secretary James G. Watt.
Mr. Watt has been painted as public enemy No. 1 by most environmentalists. Opposing his outspoken pro-development philosophy gave a renewed sense of purpose and vigor to a movement that had lost much of its luster and militancy during the Carter years.
But Feb. 21 on NBC Television's ''Meet the Press,'' the unpredictable Watt dropped an environmental bombshell. He announced that the Reagan administration will seek legislation to ban energy and mineral development in the nation's wilderness areas through the end of the century. His remarks were totally unexpected by environmentalists and the mining and oil industries, which have been key actors in the often acrimonious debate over the fate of the nation's wilderness.
Environmentalists have been arguing that the relatively small amounts of oil and gas contained in US wilderness areas do not jus-tify opening up of these areas for exploitation. Many in the oil and gas industry counter that these resources should be developed wherever they are found. Further, the industry argues that the current world oil glut is only temporary - and that once petroleum supplies become tight the US may once again be forced to consider drilling in the wilderness areas. Similar arguments have raged over the exploitation of hard rock minerals.
At the heart of this controversy is the National Wilderness System, established in 1964. Wilderness areas are not parks. They are generally remote, roadless areas set aside to be maintained in a totally natural state, ''untrammeled by man.'' This system currently consists of 80 million acres that have been designated and 61 million acres that have been proposed for wilderness.
Responding to the Watt announcement, Wiliam Turnage, executive director of the Wilderness Society agreed that ''this is clearly a major turnaround in administration policy.''
''Mr. Watt and the administration have finally realized that the public overwhelmingly supports keeping wilderness areas sacrosanct,'' elaborated Wilderness Society co-worker Gordon Roberts.
Past statements by Secretary Watt and internal Department of Interior memorandums clearly show that much of Mr. Watt's efforts in the past have been directed at ''opening up'' the public lands, including wilderness areas, to development. The Wilderness Act allows leasing for oil and gas, but no previous interior secretary had considered these activities in accord with wilderness management. As a result, there are over 1,000 lease applications pending and some 40 have been issued, although there has been no drilling as yet. Mr. Watt and his lieutenants repeatedly had promised private industry that these applications would be processed without delay, but carefully refrained from promising that they would be granted.
Despite this apparent change of heart, environmentalists are quick to point out that some provisions of the new White House wilderness policy do not suit them. One provision which environmentalists oppose puts a limit on adding new areas to the system. ''I don't think it's good public land policy to freeze the system . . . one way or the other,'' says William K. Reilly of the Conservation Foundation. This aspect of the Watt announcement led Tim Mahoney of the Sierra Club to characterize it as a ''Trojan horse.''
''The proposal by Secretary Watt seems to be an approach deserving careful study toward resolving what has been a difficult issue for a long time,'' begins the closely worded response of the American Petroleum Institute (API).
Although the API spokesman would not comment further, he admitted that the new policy came as a surprise. Environmentalists, speculating about the motive for this highly publicized new policy, believe that the controversy over exploratory oil and gas leasing in the Washakie Wilderness adjacent to Yellowstone National Park played a pivotal role in the administration's decision to take a new wilderness tact in this election year.
When Interior Department recommendations to allow leasing in this area were announced last year, Wyoming's all-Republican congressional delegation, with its strong conservative credentials, surprised Mr. Watt and the White House by publicly opposing such a policy. Just last week the Wyoming delegation introduced legislation to declare this wilderness area off limits. They are among a significant number of conservative politicians who have been advising Watt and Reagan to back off on the wilderness issue.