Collie stars in a tale of dog-napping and danger; Nop's Trials, by Donald McCaig. New York: Crown. 336 pp. $24.95.

For a border collie, ''trials'' usually test a dog's ability to perform certain herding jobs in order to bring a few unruly sheep through a designated course and into a pen. A well-trained dog, by responding to a few whistles, voice commands, and nods from a trainer, completes the trial in a given time.

Nop, however, the border collie and central character of Donald McCaig's new novel, encounters a variety of other difficulties as well, so the title takes on a double meaning.

The story is set in the rural hill country of Virginia, where Nop lives with sheep raiser Lewis Burkholder and his family.

Nop, a young sheep dog, has recently placed first in the Innesfree Stockdog Trials. Owner Burkholder is fiercely attached to the dog, who daily works along with him on the farm.

In a chance meeting in a bar, a bitter loser of the Innesfree Trials offers $ 300 to get Nop out of future competitions. During the family's dinner, on Christmas Day, Nop is dog-napped by a couple of rednecks who expect to collect the money.

When the competitor doesn't come through with the payoff, Nop is sold for a few dollars to a rodeo clown, who uses the dog's herding talents in a specialty rodeo act.

The dog escapes and attaches himself to a ''bag lady'' in a Midwestern city. When she abandons him, Nop is picked up by a dog dealer and sold to a medical lab that uses dogs to test its products. Throughout the book, the dog overcomes almost unbelievable challenges: starvation, frustration, beatings, and isolation.

Meanwhile, back at the farm, Lewis Burkholder never gives up his search for Nop. Several months later, he picks up the trail and, after a few trials of his own, is finally reunited with his dog.

Throughout the story, author McCaig holds the reader in rapt attention. Every detail of the book rings true, although the harshness of the truth behind Nop's experience was often hard for this reader to take.

While the characters and family situation in the novel are believable, it is in portraying the border collie that McCaig excels. He gives a sensitive and accurate presentation of the breed.

It's only when McCaig moves from his third-person narration into a dialogue between the dogs that things become awkward: Nop's overly quaint speech, including ''thees'' and ''thous'' in his conversation with other dogs, may distinguish his experience from that of the people in the story, but the quaintness seems out of character.

Never mind, though. ''Nop's Trials'' is an enjoyable read for dog lovers and general readers alike. After all, how long has it been since a dog was the central figure in what promises to be a ''best selling'' novel?

Sonia W. Thomas is on the book page staff.