For the next 10 days, the only power put down on the roads in Yellowstone National Park will be put there by people. The only fuel burned will be calories. The only congestion will be caused by buffalo blocking the roads. The only noise will be grunting bison, honking geese, whispering pines and loud expressions of excitement.

Between now and April 17, open roads in the park will be limited to people on bikes or rollerblades or in hiking shoes. With the exception of routine park service traffic, no motorized vehicles will be allowed in the park.Not since the early years of Yellowstone has there been such an opportunity to see it in such a natural state . . . so quiet, so alone, so undeveloped, so primitive.

On April 17, a stream of cars and trucks will begin driving into the park. They will keep coming until the roads are covered with snow. Then the snowmobilers will start arriving. They will stay until crews begin work to uncover the roads for the cars and trucks.

For a brief three-week period during the changeover from winter to summer, the park is vacant. No people, no cars. The roads are clear, the scenery is just coming into its full summer beauty, and the animals are feeding and feeling good.

"We were looking for ways of extending the use of the park," said Clyde Seely, owner of Yellowstone Tour and Travel and the Holiday Inn in West Yellowstone. "I was at a travel convention in Georgia. On a whim I wrote on a piece of paper, `Ask about biking in Yellowstone with no cars,' and taped it to a table. We had more people stop and ask about the biking than about anything else.

"We came back, talked with park officials, and they agreed to open the park to bicycles the three weeks before they opened to vehicle traffic."

Yellowstone opened to bicycles, bladers and hikers on March 24. Because of an on-going bear study, the section between Madison Junction and Old Faithful could not be opened. But bikers and hikers can get into the park, without a fee, from the West Yellowstone or Mammoth Hot Springs entrances. People, under their own power, can then travel freely between West Yellowstone, Madison, Norris, Canyon Village and Mammoth, which accounts for about 61 miles of paved road within the park.

The rules are simple. Ride as you would on any highway, be prepared for all situations, and animals have the right-of-way. More so now, in fact, than at any other times. In the summer, visitors are in their vehicles. In the winter, bison are lethargic and slow because of the lack of food and the cold. In the spring they are eating regularly, are warm and tend to be more active. They'll trot and grunt in the spring, something they almost never do in the winter. Other animals in the park, like the elk, eagles, coyotes and geese, are noticeably there in the spring, but seldom are the chance meetings as up-close and unnerving as with the buffalo.

The one thing people have to be prepared for, warns Seely, is the weather. It can be sunny and comfortable in the spring, as it was last Saturday, "or we can get the sudden little storms. They usually don't last long, and then things warm back up. But people need to be prepared for all conditions."

Last weekend, the park drew about 50 bikers. Most rode in twos, but there were several families that included young children and even younger passengers in tote-cars pulled behind a parent's bike.

The fact that roads in the park rise and drop gradually, and for the most part are level, makes it possible for people of varying skill levels to ride in the park, said Seely, "They can go as far as they want for a long as they want, and because there are no steep climbs or great elevation changes, they can make it back without much difficulty."

Seely said the town of West Yellowstone has long had the idea of developing biking in the area. Along with the roads, "there is a lot of mountain biking terrain available. All of the snowmobile trails, and we have hundreds of miles of snowmobile trails, as well as the cross-country trails, are open to biking. We have more than 50 kilometers of trails at the Rendezvous Cross Country Center."

Because the park will not allow organized trips, and to show visitors some of the biking opportunities in the area, the town will hold its Yellowstone Spring Cycle Tour on Saturday. This is an organized ride outside the park that will cover 65 miles and pass three landmarks - Hebgen, Quake and Henry's lakes.

Riders will start at 9 a.m. and head north out of town with a police escort. There will be several feed stations along the route offering snacks and drinks. There will also be a truck and trailer following the group, said Seely, "To pick up anyone along the way. Riders will be able to go as far as they want, then put their bike on the trailer and ride back to town."

At the Quake Lake visitor's center there will be an interpretive program on the famous earthquake of 1959 that hit the Yellowstone area and, because of the resulting landslide, formed the lake.

The registration fee - $25 for adults and $15 for children - will cover costs of the truck and trailer, feed stations, evening spaghetti dinner and entertainment.

Those riders who would like to ride into the park can do so on their own on days before or after the organized ride - for the next 10 days, anyway. Then the cars start arriving and the peace and solitude will be broken.