WASHINGTON – Ladies and gentlemen, start your engines.

The Bush administration plans to allow more snowmobiles in Yellowstone and Grand Teton national parks on average, while cutting numbers on the busiest days. The decision reverses one taken during the Clinton presidency that would have banned them by next winter.

There would be no limits on snowmobiles for the winter season beginning next month and running until mid-March, Interior Department officials said.

But starting in December 2003, no more than 1,100 snowmobiles a day would be allowed in the two popular Western parks together and a portion of the John D. Rockefeller Jr. Memorial Parkway connecting them, the officials said, speaking on condition of anonymity.

For the past decade, the parks have had an average of 840 snowmobiles daily during the winter but up to 1,650 a day during holiday and other busy weekends. Both parks are in northwestern Wyoming, but Yellowstone also extends into Idaho and Montana, from where most snowmobilers enter the park.

The Interior Department planned to release an environmental impact statement Tuesday that details the proposal. The ceiling represents a compromise between unlimited access wanted by snowmobile makers and users and the ban sought by environmental groups and some Democrats in Congress.

“This is just a boon to the industry,” said Kristen Brengel of The Wilderness Society, an environmental group. “This is not what the American public has been expecting.”

Bill Dart, public lands director for Idaho-based Blue Ribbon Coalition, which advocates opening more public lands to recreational motor vehicles, said his group is satisfied with the peak-days ceiling, even though it might not reflect the rising popularity of snowmobiling in the parks in recent years.

“Clearly I don’t think they’re caving to industry,” Dart said. “They’re talking about one-third less numbers on peak days.”

To minimize the impact and maximize safety, the regulations would require that 80 percent of the snowmobiles allowed in the two parks be led by commercial guides. Also, beginning next year commercially rented snowmobiles would have to have four-stroke engines, which are said to be quieter and less polluting. Private snowmobile owners could use traditional two-cycle engines until the 2004-2005 winter season.

No more than 950 snowmobiles would be allowed into Yellowstone National Park: 550 through the West entrance and 50 through the North entrance, both in Montana; and 250 through the South entrance and 100 through the East entrance, both in Wyoming.

Another 75 would be allowed into Grand Teton National Park through a snowmobile trail along the Continental Divide and 75 more from a road along the Rockefeller parkway.

The Interior Department officials said their plan is based on a belief that four-stroke engines can significantly cut noise and reduce emissions of hydrocarbons by 90 percent and carbon monoxide by 70 percent.

They left open the possibility of adjusting the caps based on results from air quality and noise monitoring stations that will be installed in the parks. University and state contractors will be hired to collect data on air quality, noise and the effect on wildlife.

“This plan, in essence, stays away from the extremes,” said Eric Ruff, an Interior Department spokesman. “It strikes a good balance. It protects resources and allows visitors a unique experience. It’s never been managed like this before.”