Oh, deer! Here come some more cross-country skiers!

Question: Which disturbs wildlife more? A cross-country skier or a snowmobiler? The answer has a bearing on a political debate over what winter activities should be allowed in US national parks.

Most people will probably pick the snowmobiler as most disruptive. After all, the common image of the snowmobile, ripping apart the snow-blanketed silence of a winter forest with its whining engine, is far more disturbing than that of a cross-country skier gliding quietly along.

But experts say it is not at all clear an animal would agree.

''It's a matter of observation, but I would say that cross-country skiers disturb the animals far more than snowmobilers,'' says Pete Hayden, naturalist at Grand Teton National Park in Wyoming. ''Animals begin moving away the minute they spot a skier, while a snowmobile has to get pretty close before they react.'' His opinion is backed by the few formal studies that have been conducted on the matter.

Game animals appear to associate cross-country skiers with hunters while lumping snowmobiles with cars, a much lesser danger, the park naturalist surmises.

In Yellowstone National Park, where officials have just opened the Mammoth Hot Springs area to cross-country skiing, the rangers are keeping a close eye on the effects on wildlife. They are concerned because during the winter animals don't have much energy to flee from human intruders.

The question of the relative impact of cross-country skiers and snowmobilers on wildlife is just one facet of a debate over the proper winter uses of a number of national parks. The debate also turns on other possible environmental impacts from snowmobiling, the potential for disturbance of other park users, and the reasons why the parks were formed in the first place.

Conservationists are generally pro-cross-country skier and anti-snowmobiler, while influential members of the Reagan administration, such as Interior Secretary James Watt, are vocal in their support of snowmobiling.

Last month the Park Service announced it was opening areas in five parks to snowmobilers: Olympic National Park in Washington state; Herbert Hoover National Historic Site in Iowa; Perry's Victory and International Peace Memorial in Ohio; Pictured Rocks National Lakeshore in Michigan; and, St. Croix National Scenic Riverway in Wisconsin. This increases to 29 the number of areas in the park system open, to some extent, to snowmobiling.

Under current national policy, which has been in effect since 1979, snowmobiles ''may be permitted in units of the National Park System as a mode of transportation to provide the opportunity for visitors to see, sense, and enjoy the special qualities or features of the park in winter.'' This policy restricts snowmobiles to roads and frozen lake beds, areas where motorized transportation is allowed during the summer.

The one exception to this rule is in Grand Teton. As part of a 1980 compromise, snowmobilers are allowed the run of 16,000 acres in an area known as the Potholes after most of the large animals have moved out and when there is adequate snow cover.

At heart, the debate is philosophical.

''The primary purpose that the parks serve, in addition to preservation, is to educate and to inspire the people who visit,'' says Destry Jarvis of the National Parks and Conservation Association. ''They are not intended to serve as simply a source of recreation.''

Basically, the conservationist argues that activities such as snowmobiling, downhill skiing, or airboat trips in the Everglades tend to be thrill-seeking rather than a means by which nature can be fully appreciated.

Snowmobilers denounce this attitude as elitist. ''The environmentalist crusade is largely a movement among the affluent, well-educated, white upper-middle-class elite,'' charges the International Snowmobile Industry Association (ISIA) in a paper ''Snowmobiling and Our Environment: Facts and Fantasies.'' The industry trade association argues their machines allow all types of people to enjoy the beauties of nature.

ISIA also argues that the common image of the snowmobile is outdated. Today's machines are much quieter than those being made in the 1960's. They also claim that properly operated snowmobiles have no adverse environmental impacts.

But Mr. Jarvis remains concerned about the small percentage of snowmobilers who do not behave responsibly. ''The fact remains that the threshold of environmental impact for any motorized vehicle is much lower than for people afoot. The problem with any permissive policy is that you have to be able to enforce it. The park service is unable to enforce the snowmobiling policy,'' he says.