Mention the national parks and the images start to flow: snow-capped mountains, virgin forests, limitless horizons.

Here's another way to think of them: profit centers.

According to an Interior Department investigation released yesterday, park concessionaires in 1988 recorded $500 million in gross profits while returning $12.5 million -- 2.5 percent of the total -- to the U.S. Treasury.

Investigators told a congressional panel that Interior is losing tens of millions annually as a consequence of long-term contracts granting virtual monopolies to the operators of campgrounds, hotels, restaurants and other services on the nation's 355-unit park system.

The most profitable of those companies is Yosemite Park and Curry Co., a subsidiary of giant MCA Inc. that operates commercial services in California's Yosemite National Park and recorded gross 1988 profits of $76,570,000. The fee paid to the government was $570,000 -- about three quarters of 1 percent of the total.

"Our national parks were established for people, not profiteers," said Rep. Bruce F. Vento (D-Minn.), chairman of the House interior subcommittee on national parks. "They are to be preserved and protected -- not prostituted either for private interests on in the name of the budget."

The investigation was ordered by Secretary Manuel Lujan Jr., who wants to increase the fees charged to the concessionaires, shorten the duration of the contracts and plow the extra money back into the parks. Chief among his concerns is that some concessionaires have been allowed to own park facilities, which would have to be purchased for millions if their contracts were canceled.

Lujan's proposals have been bitterly resisted by the concessionaires, some of them owned by large conglomerates, which went to court on Wednesday in an attempt to withhold portions of the investigation from release.

"I think there's definitely a smear campaign because of our size and our success," said Ed Hardy, president of Yosemite Park and Curry Co.

Hardy said the debate over Curry Co.'s franchise fee overlooks the $100 million spent by the company on improving facilities that benefit park visitors, and the resources it has devoted to cleaning up back-country areas and establishing recycling programs. "You have to look at the total package," he said.

Many concessionaires, Curry among them, have operated in the national parks for decades. In the early 1960s, anxious to attract visitors and provide services where none existed, the government signed 30-year contracts at favorable terms. But as profits have soared along with the number of visitors, Interior officials have become concerned that the concessionaires are getting a free ride.

The investigation by Interior Inspector General James R. Richards found that if franchise fees charged to concessionaires had conformed to industry norms for services such as hotels and restaurants, the National Park Service would have reaped an additional $83 million between 1984 and 1988.

Much of the information relating to specific companies and contracts was deleted from the inspector general's report under an agreement reached Wednesday between Interior officials and attorneys for the concessionaires. But a source with access to the uncensored document provided several key examples of profitable park concessions.

Among them are Amfac Hotels and Resorts (Grand Canyon), which recorded 1988 profits of $44.3 million and paid a franchise fee of $1.2 million; ARA Leisure Services Inc./Wahweap Lodge (Glen Canyon National Recreation Area), $22.5 million and $505,000; Guest Services, Inc. (National Capital Region), $17 million and $480,000; and Hamilton Stores (Yellowstone), $13 million and $360,000.

The report also found that, while concessionaires often are responsible for major physical improvements in national parks, they also reap the greatest benefits from those expenditures. For example, one Yellowstone concessionaire agreed to spend 22 percent of annual revenues on maintaining government-owned buildings, but used some of the money to buy vending machines, snowmobiles and other "revenue-producing" items, according to the report.

In theory contracts are open for competitive bids when they come up for renewal, but a variety of obstacles stand in the way of would-be concessionaires. Among the biggest is the right of concessionaires to own buildings and other facilities inside park boundaries, which makes them extraordinarily difficult to dislodge.

But the Curry Co.'s Hardy said no business would make an investment that did not include the right to build up equity. "Interwoven into the contract concessions is the incentive" to invest in the parks, he said.