The Bush administration issued a new rulebook for managing the national parks yesterday, jettisoning some changes it had considered instituting earlier this year that were criticized by park advocates.

The roughly 200-page draft "management policy document" guides park supervisors on everything from protecting homeland security to allowing off-road vehicles to traverse nature areas. Park advocates had argued that an earlier version of the document, drafted by Deputy Assistant Interior Secretary Paul Hoffman, would undermine environmental protections.

Park Service Director Fran P. Mainella said at a news conference that the rules reflect the agency's attempt "to be 21st century-relevant. . . . To manage parks emphasizing either conservation or enjoyment, to the exclusion of the other, imperils the national park concept."

Environmentalists lauded the administration for abandoning some of the proposed changes that became public in August, including language that instructed park officials to try to mitigate air pollution rather than eliminate it. But they questioned other aspects of the new plan -- now subject to a 90-day public comment period -- citing language that makes good visibility in national parks an "associated value" rather than a "highly valued" resource.

"There are some troubling signs there, even as some improvements have been made," said Mark Wenzler, clean air director at the National Parks Conservation Association, an advocacy group. Wenzler added that the new draft language may "allow a level of pollution that would not be acceptable under the current policies."

Steve Martin, the Park Service's deputy director and a 30-year agency veteran, said the administration is open to changing the document if the results might damage the environment.

"We're looking for those kind of comments, if we've hurt something inadvertently," he said of Wenzler's criticism. Martin added that the earlier draft was aimed at sparking a vigorous internal debate: "It was not ready for prime time."

The proposal would not allow for more snowmobile or off-road vehicle use in national parks, Mainella said, but would provide guidelines for such activities in areas where they are already taking place.

Administration officials hope to block activities that could degrade the parks, she said, adding: "We do not want to go all the way to impairment before we put the brakes on."

However, some park advocates -- who noted they were still trying to analyze the lengthy document -- questioned why the administration decided to rewrite the guidelines, which were revised in December 2000.