Framed against snow-streaked mountain pinnacles that have towered here for 9 million years, Park Ranger Tim Bywater welcomes tourists to Grand Teton National Park at 3 p.m. each day with these words: "In Teton, we're walking into the past."

And like clockwork, right in the middle of Bywater's introduction, a Frontier Airlines jet roars over the aspens and lodge pole pines toward Jackson Hole Airport--a thundering reminder that even here, the wonders of nature must vie for space with the works of modern man.

The airport lies within Grand Teton. So do dozens of private houses and a 70-year-old dam that is threatening to fail. Roads are being rebuilt and parking lots repaved, all to accommodate thousands of people and cars that arrive here each week. Just outside the park are a shooting range, houses, condominiums, oil wells and more.

Every national park has a breaking point, where the delicate balance between the rhythms of nature and the works of man can be irreversibly tipped, where the natural world inside the park resembles more and more the world outside.

To visitors who gather around Bywater every day, this process is so gradual that it is undetectable. But to those who know Teton and other national parks intimately, it is alarming.

"The impact on wildlife of one more residence or one more road or even one more subdivision may not be significant and usually cannot be measured," a team of biologists and government officials recently wrote of Teton and the neighboring National Elk Refuge. "But the cumulative impacts of more and more development and disturbance are relentless and eventually disastrous."

The Jackson Hole Cooperative Elk Studies Group's comments came after Interior Secretary James G. Watt imposed a near-moratorium on buying new park land in 1981.

Watt said the National Park Service governs more land--79 million acres in 335 units--than it can care for. It was more important, Watt said, to repair aging park facilities, which he said had fallen into "shameful" disrepair under his predecessors. He said that the task would cost $1 billion over five years and that the government could not afford to expand parks and repair them at the same time.

"We are not going to be measured in our success on how many acres we added" to national parks, Watt said in a speech last week. "We will be measured on how did we take care of that which was given to our charge."

Congress did not draw Teton's present boundaries until 1950, and within its 320,000 acres were large pockets of private property. Today, more than 3,000 acres remain to be acquired.

Under official Teton policy, privately owned parcels may not be developed without National Park Service permission, and unauthorized construction is to be condemned. But since 1981, there has been some private development:

* Five houses have been built on pockets of private land near Teton's southern and eastern boundaries. Interior has not moved to condemn them.

* In the National Elk Refuge south of the park, a small subdivision is being developed in the wintering range of 6,000 elk. The landowner had offered to sell the land to Interior, but agency officials said there was no money to buy.

* A draft of a new plan for the park would allow construction of as many as 74 new houses in the southeastern corner of Teton along the elks' route to the refuge.

* A local family has proposed building a 160-acre subdivision near a scenic butte.

Interior officials said they are not acquiescing in unauthorized development of the park. They said they are considering a buy-out of the proposed subdivision, which Park Superintendent Pete Stark said "would be rather disastrous to park values." And Interior recently acquired 1,200 acres of private property in Teton by swapping it for federal lands outside the park.

Stark said he and most of his staff viewed the new houses as a worthwhile trade for the repairs. Money that might have gone into condemnation was budgeted for bricks and mortar.

"There will be 3.5 million visitors here this year, and not one of them will go home with a complaint about those houses," he said. "At some point, you have to repair the roof on this building. I believe the American public wants us to do that."

While insisting that their park is not in "shameful" condition, Teton officials nonetheless came up with a list of long-deferred maintenance projects and received more money this year under Watt's face-lifting progam ($3.6 million) than for their entire operating budget ($3.2 million).

They will repair 7 1/2 miles of pot-holed highway ($2.3 million), repave cracked parking lots around hotel concessions ($300,000), replace a leaking water line ($200,000), repair the leaky ceiling in Stark's office and the visitor center ($40,000) and more.

Stark said the proposal to allow new development near the elks' migration route is a concession to reality. The area, known as Kelly, is a 70-acre town here before Teton's boundaries were drawn around it in 1950. The park service has spent several years and $750,000 trying to buy it out, but 34 homes, a post office, a school and a grocery store remain. It is unclear when, if ever, the buy-out can be finished, he said.

"The elk can go around it, is my theory," Stark said.

Some biologists have a different view. An official of Interior's Fish and Wildlife Service said: "We always say elk will go around man and go around houses. And they will, until there's no place to go around. The question is, when do you call a halt?"

Park officials said they have too little research data on Teton's wildlife to answer that question. The park's research budget has remained flat for several years.

But the possibility of development is only one of the threats to Teton, according to park officials.

An earthen dam that since the 1920s has impounded scenic Jackson Lake, one of the most popular spots in the park, is in danger of collapse, according to Interior's Bureau of Reclamation.

The bureau maintains the best technical solution is a new dam about 3 1/2 miles downstream, flooding 2,000 acres of Teton's most sensitive wildlife habitat, Oxbow Bend, home of elk, moose, great blue herons, pelicans and at least one bald eagle.

Rebuilding the present dam would cost more and disturb less, the bureau said, but even that would require moving tons of construction materials and machines into the lake area. Doing nothing could lead to a flood that would engulf $120 million of property and about 2,000 people, the bureau said.

The airport, with six jet flights daily and constant commuter-plane traffic, appears likely to be in operation indefinitely. Watt's predecessor, Cecil D. Andrus, had ordered the airport to close in the 1990s when its previous permit was to expire. But Watt recently extended the permit for 50 years, while requiring stronger noise controls.

For years, snowmobilers have cruised off-road areas of the park, although the practice was forbidden in all other parks outside Alaska and was described in a 1980 report by Teton officials as "contrary to appropriate use of park resources." Earlier this year, Interior issued a regulation formally authorizing off-road snowmobiling in Teton as an exception to the ban elsewhere.

Park scientists said they have documented no damage to wildlife from snowmobiling and that no data indicates that airplane noise takes a toll. The same goes for a handful of houses.

Teton biologist Pete Hayden spoke of these changes as he looked out on a bend in the Snake River, surrounded by willows, blue spruce, cottonwoods, bluegrass and dogwoods. The exquisite blue-gray mountains rose in the background, and waterfowl dotted river marshes.

From this spot, there was no hint of outside pressure on Teton's natural world. But Hayden said he could feel it.

"Twenty years down the road, this park will have more roads, more houses and less wildlife," he said. "Who's to say one house has an impact? Maybe it won't. But at some point, it will."