The "sold out" sign is up on Nepal's mountains, which include eight of the world's 10 highest peaks.

Mount Everest, the world's tallest mountain -- which was conquered for the first time just 27 years ago -- is by far the most popular. It is booked solid, two expeditions a season, through 1987 and the Nepalese government has received enough applications to take care of the rest of the decade.

Nepal's six other peaks higher than 26,700 feet are booked through 1983.

"If you want to climb them, you have to wait until 1984," said Shailendra Raj Sharma, the Nepalese government official in charge of mountain-climbing expeditions.

Mountain-climbing has become so popular that Nepal found in 1978 that it had to regulate expeditions to make sure they don't bump into each other, often with disastrous results, on the peaks.

The growing popularity of mountain-climbing as a sport means big bucks for Nepal. Last fall alone the country earned $1.2 million in much needed foreign exchange from the fees paid by 26 international mountain-climbing expeditions.

Tourism -- in which mountain-climbing and its less strenuous cousin, trekking, plays a big part -- has now replaced the hiring of Nepal's famed Gurkha troops in the Indian and British armies as the largest source of foreign exchange.

"There's been an explosion in interest in mountain climbing," said Al Read, a former Foreign Service officer who traded striped pants for hobnailed climb boots to promote mountain travel here.

"People are no longer content to see the museums of Europe when they can go mountain-climbing, trekking or scuba diving. More and more people are coming to the Himalayas for the kind of adventure 10 years ago they only read about in books or dreamt about."

Part of the new interest in climbing, said Read, is the result of improved equipment that makes it less dangerous for ordinary people.

Nonetheless, it is still an expensive sport. It costs, for instance, $12,000 in license fees alone to climb Nepal's other peaks higher than 26,700 feet.

But compared to India and Pakistan, Nepal is the "cheapest," Sharma said.

According to sources here, China has decided to open some of its peaks to expeditions and has studied how Nepal regulates climbers. "The Chinese decided to double the fees," said one source.

But fees are only one aspect of income from mountain-climbing. It costs an average of $500,000 to mount a major mountain-climbing expedition, and Sharma estimated about half the money gets spent here.

"The money goes to the lowest strata of people in the country," Sharma said. Expeditions buy food and hire Sherpa guides in the villages. Moreover, under Nepalese law they must outfit the native porters with the same cold-weather gear the climbers themselves wear. The porters generally sell these sleeping bags, parkas and other equipment after every expedition, giving them additional income.

To meet the demand for mountain-climbing time, Nepal has opened some of its peaks for the winter, when the frigid cold makes the climb extremely tough. But, said Sharma, the snows are more constant and there is less risk of a killing avalanche.

Last month, a group of Poles made the first winter ascent of Mount Everest.

But styles in mountain-climbing are changing. When mountaineers first began climbing the Himalayan peaks, they staged all-out assaults going up the easier faces and establishing one or more base camps before the final attack on the summit. One Italian military expedition even used helicopters to get up Mount Everest, one of which crashed and is now buried beneath a glacier. Needless to say, true mountain climbers considered that unsporting.

Now small group-climbs up the more difficult faces are the rage, with no Sherpas above a base camp. "Just you and a couple of friends," said Elizabeth Hawley, an American who provides logistical help for climbs.

"It's style and difficulty now, not just height," added Read.

Probably the most stylish climber of them all, Tirolean mountaineer Reinhold Messner, who was the first to climb Everest without oxygen, plans to conquer it alone -- from the Chinese side, not from Nepal -- next June.