PASADENA, CALIF. -- Rusty, a multi-colored collie of uncertain parentage, loves pickup trucks. Even in this teeming California suburb, she adds a vivid, enduring image of the great American West as she stands tall and proud, the breeze rippling her fur, in the back of her owner's blue Toyota 4X4.

But Rusty's high-riding days are numbered. And this is not just a matter of some particularly bad bump on some future road.

With a suddenness that has delighted Humane Society officials, worried weekend truckers and appalled political conservatives, the state of California appears on the verge of outlawing untethered animals in the backs of pickup trucks. Such a breathtaking assault on Americana has not gone entirely unchallenged, but a clever compromise with farming interests, a growing number of county dog-restraint laws and a mysterious report of 100,000 annual animal deaths seem to have carried the day.

Once victory in California is assured, the anti-dog-in-truck forces anticipate taking their message to the rest of the country, where reaction has been mixed. Washington state and Oregon have passed similar dog-restraint laws, according to Barbara Cassidy of the Humane Society of the United States. One restraint provision died in the Maryland legislature. Colorado legislators killed one after hearing sarcastic predictions of further proposals for doggie crash helmets.

After rejecting several attempts, the California Assembly passed its dog-in-the-pickup bill last month, 50 to 19. The state Senate appears likely to follow suit. California pickup owners and their dogs are resigning themselves to a severe change in life style.

Jodie Steck, a wire-service editor who lives in South Pasadena, said she has discussed the proposed tethering law with Max, her seven-year-old black Labrador-Irish Setter. "Max says if it comes to that, he would prefer to ride in the front with me," she said, even though she has noticed that he is distinctly uncomfortable up there.

Since being given Rusty -- a shelter foundling -- two years ago, restaurant manager Gregory Holmes has been tying her with a 5-foot leash to the back of the truck cab. Rusty has enough rope to roam the truck bed, as she likes to do, or to leap over the side and land on the ground if such a foolish notion possessed her.

Under the proposed California law, that arrangement would be insufficient. The leash would have to prevent the animal from leaving the truck. Otherwise, a dog must be carried in a closed container or in the cab.

Donna Wetterer Pane, legislative assistant to the bill's sponsor, Assemblyman Jack O'Connell, a Democrat from Santa Barbara, said their office received only one negative letter on the bill.

In the northern part of the state, where Assemblyman Stan Statham, a Republican from Redding, represents the huge, thinly populated First District, the bill has been intensely unpopular. Statham said he voted against it because there was no demonstrated need. Moreover, it threatens his constituents' life styles. And he thought that the bill "had crossed the line" of government interference in private conduct.

Holmes, who named his dog after the Rusty Scupper restaurant he manages, grew up in Colorado where dogs in pickups were plentiful. He supports the use of restraints but does not know how he would apply the new law to Rusty.

Steck calls the restraint bill intrusive and unnecessary. "The dogs are real stable back there," she said, "but they have to be able to move around on their four legs." A previous dog of which she was less fond once leapt from her truck before it had stopped completely but was unharmed. "She was too retarded to notice, anyway," she said.

To win at least the neutrality of farm and ranch groups, O'Connell agreed to include an exemption for livestock or "a dog whose owner either owns or is employed by a ranching or farming operation." The law applies only to highway driving, but covers farm and nonfarm dogs riding on an urban freeway with a 55 mph speed limit.

The bill won a great boost from a widely seen Los Angeles broadcast by television consumer advocate David Horowitz, introduced with the statement that "100,000 dogs will be killed or seriously injured this year riding in the back of open pickup trucks." Horowitz's researcher, Gloria Drexler, said the figure was provided by the San Diego Humane Society, which said it was obtained from the American Humane Association in Denver.

Rich Meyer, an association staff member, said he took the figure from an official of the Animal Protection Institute in Sacramento. That official recently resigned and could not be reached. "I am very leery of that figure," said Charlene Drennon, director of the Humane Society of the U.S. West Coast.

In 1982, the legislature ordered the California Highway Patrol to determine how often animals riding in the back of trucks cause accidents. Three such reports were discovered in three years, Highway Patrol spokeswoman Susan Cowan-Scott said. She noted that the agency has never tried to determine how many animal deaths and injuries result from pickup riding.

Vicki Young, an agent with the Los Angeles office of the Society for the Prevention of Cruelty to Animals, said she is certain there are many such animal fatalities, most unreported. Last week, she saw a small German shepherd scrabbling back and forth frantically on the bed of a pickup as it negotiated the twists and turns of Little Tujunga Canyon. Several times the dog nearly slipped out.

But when the driver happened to stop and she spoke to him, he seemed "very perturbed" at her mild lecture. "He didn't think it was any big deal," she said.