The new chief of the National Park Service said today that he will shut the gates of some major parks during busy vacation periods to restrict access and reduce human impact on the parks' original inhabitants.

Facing increasing conflict between his agency's two missions -- natural preservation and human recreation -- the new director, William Penn Mott Jr., said, "We've got to err on the side of preservation."

"Cattlemen know how many cattle you can graze on an acre of land without lasting damage," Mott said. "It's called 'carrying capacity.' Well, we've got to know the carrying capacity of our parks and start saying, 'After this, nobody gets in this weekend.' "

Mott listed Yosemite and Sequoia-Kings Canyon National Parks in California as examples of parks with too many visitors. Park Service officials said other likely candidates for limitation would be Great Smoky Mountains National Park in Tennessee and the Yellowstone and Grand Teton parks here.

In 1960, according to the Census Bureau, there were 26.6 million visits to the national parks. In the quarter-century since, visits have more than doubled. In 1983, the parks logged 63.8 million visits.

Park Service professionals and conservation groups have discussed limiting access to the national parks. But the government has not imposed the direct entry controls Mott plans to implement, instead trying indirect controls such as limiting the number of inns and campgrounds within a park.

Limiting visitors can become a hot political issue not only with tourists who are turned away but also among people who earn their livelihood from the tourist trade in nearby communities.

But Mott, a white-haired, red-faced 75-year-old Californian who seems as chipper and as optimistic as the 73-year-old president who just appointed him, said the public "absolutely will" accept controls, even if it means turnstiles and counting machines at the park gates.

Mott said entry limits were imposed experimentally at Yosemite two weeks ago "and nobody complained -- well, one angry letter."

Mott's firm assertion that his agency must strike the balance so as to protect the parks' natural ecosystems won fervent approval from conservation leaders who came here to meet with the new director just a week after he took office.

"Mr. Mott is quite possibly the best appointment from an environmental standpoint that Ronald Reagan has made," said Phillip Hocker, treasurer of the Sierra Club.

Mott headed the California State Park system when Reagan was governor. He is a trustee of the National Parks and Conservation Association, one of the groups that has been most critical of the administration's environmental policies.

Mott worked as a landscape architect for the Park Service during Franklin D. Roosevelt's administration. He says he will replace at least one top political appointee in the service with a career executive.

To emphasize his respect for Park Service veterans, Mott arranged for his first official trip as director to be this one to Yellowstone, where he presided over a ceremony honoring employes.

Mott said his plan to shut the gates when parks reach "carrying capacity" reflects the need to provide a "quality outdoor experience" for park visitors.

"When you've got so many people in Yosemite that the roads look like the Golden Gate Bridge at rush hour and there's more smoke than in the heart of the city, that's not what people are coming to our parks for," he said.

Further, he said, the Park Service has to worry about long-range environmental damage from intense human use. "There are some areas, some parks, where we've got greater concentrations of people than the natural system can support over the long run," he said.