WASHINGTON (AP) _ It's harder work being a snake than scientists once believed.

Three University of California, Irvine, scientists studying the energy cost of locomotion report that it takes as much energy for a snake to undulate along the ground as does for an animal with legs to walk.

This, says Bruce Jayne, a coauthor of a study published today in Science, is contrary to what scientists have long believed.

''Our findings do not support the widely held notion that the energetic cost of terrestrial locomotion by limbless animals is less than that of limbed animals,'' he said.

In plain language, it is as hard to wiggle along as it is to walk along. Or it takes as much energy to slither as it does to stroll.

To find out how hard a snake has to work to get from one place to another, Jayne and his colleagues got a group of black racer snakes and figured out a way to put them through an aerobic workout.

First, the three-foot-long snakes were fitted with little clear plastic masks. Leading from each mask was a hose that would collect the exhaled reptilian breath.

Then the snakes, one by one, were put on a treadmill - or, perhaps, it was a slithermill.

Anyway, once placed on the device a snake crawled like crazy to keep up with the surface moving under it. As it worked, the snake's exhaled breath was drawn through the hose and into a machine that measured the oxygen content.

How much oxygen was depleted from the breath, said Jayne, was a measure of the amount of energy the snake needed to slither along.

The scientists then compared the results with similar studies of limbed animals, such as lizards, of the same mass, or weight. The energy used by both the footed and the fanged was about the same, they discovered.

Jayne said the study casts doubt on a long-held notion about the evolution of limbless land animals, such as snakes and some legless lizards.

''One explanation for the evolutionary loss of limbs has been that it was energetically less expensive,'' he said. Scientists, said Jayne, have suggested that being legless gave some animals an advantage because it was thought to require less energy to move like a snake than to walk on legs.

Now, he said, that theory no longer seems valid.

Jayne said the undulating motion of a snake on a flat surface is only one of several ways the reptile can move.

Another method is used when a snake crawls through a tunnel wider than its body, but not wide enough to slither.

For such a place, snakes use what is called a concertina motion.

This movement involves bunching up the lower body and then extending the head forward. Then the upper body is bunched up and the tail pulled forward.

To test the energy need for this motion, Jayne and his team built a tunnel treadmill and the snakes worked out as before with their exhaled breath measured.

''Concertina locomotion is very expensive (in energy use),'' said Jayne. He said it was ''far and away more expensive than virtually all other kinds of locomotion.''

Jayne said the black racers used in the experiments lived up to their names. In a short bursts, he said, the snakes were able to slither at more than four miles an hour. Typically, though, they worked at a speed of half a mile an hour, or less. For the concertina locomotion, the typical speed dropped to less than 200 feet an hour.

The scientists got their test subjects from an animals supply house, but black racers are common snakes in the southeastern United States. They eat small rodents, birds and lizards and typically keep away from humans. They are not poisonous, but, as Jayne learned, they can bite.

''Sometimes when I would get them out of their cages, they would nip me,'' he said.

How do you get a snake to start a treadmill workout?

''You could just touch their tail or wave your hand and off they'd go,'' said the scientist.

Jayne's coauthors in the study were Albert Bennett and Michael Walton. Science, which published the study, is the journal of the American Association for the Advancement of Science.