The most serious in a rash of western fires, including a California blaze threatening hundreds of homes, appear to have been set by arsonists who may be attracted by the widespread news coverage, fire officials and mental health experts said today.

"In our experience, the more fires you have the more arsonists you get," said Bill McCleese, assistant director of fire management for the U.S. Forest Service. "A bad fire season brings out the kind of people who start fires."

Forest Service officials in Washington called the outbreak the worst in eight years, with more than 1,000 fires destroying nearly 1.4 million acres of brush and timber in the United States and western Canada. With an unusually dry spring giving the normal fire season more than a month's head start, the nation could be headed toward the worst fire year of the century, the officials said.

The latest and most dangerous of the major fires, a blaze near Los Gatos, Calif., has forced 4,500 people to evacuate. "Our investigators have eliminated all possible natural causes," said Jim Bliss of the California Division of Forestry. "We believe it is arson."

Rain fell this afternoon in Los Gatos, boosting firefighters' efforts.

Other major fires blamed on arsonists include the Wheeler Canyon fire that has destroyed 89,200 acres near Ojai, Calif., and the Baldwin Hills fire, which killed three persons and destroyed 48 homes in a Los Angeles neighborhood after witnesses saw an incendiary device thrown from a car into nearby bushland.

In areas such as the Santa Cruz mountains, where the Los Gatos fire has destroyed 14,000 acres, the denuded land is expected to give way to serious mud slides if heavy rain falls this winter. The destructive cycle of fire and flood has become a pattern in many areas where Californians have built fashionable homes on forested or grassy hillsides.

The U.S. Forest Service has increased to $10,000 its reward for information leading to the person who set the Wheeler fire east of Santa Barbara. "It's very difficult to prove arson because you almost have to see the person starting it," McCleese said.

Dr. Kenneth R. Fineman, professor of psychiatry at the University of California, Irvine, said the excitement created by newspaper and television coverage of major fires is often irresistible to arsonists. "The copycat syndrome is very real," he said.

"Unfortunately in this case, they learn the most from modeling," said Joel Dvoskin, forensic services director for the New York state office of mental health. His studies of arsonists suggest that fires set by copycats tend to be larger and more serious than those set by individuals striking out at some perceived enemy.

The key element, Dvoskin said, is anger. Arsonists are usually emotionally disturbed and unable to deal with their rages, he said, "so they deal with the stress with the primitive resource at hand -- fire," he said.

According to federal officials, arson killed 1,500 people in the United States last year, injured 14,000 and damaged nearly $20 billion in property. The toll has increased tenfold in the last 15 years.

McCleese said the Forest Service estimates that about 17 percent of the fires it handles are set by arsonists. The other causes are: lightning, 47 percent; careless camping, 12 percent; smoking, 7 percent; trash burning, 4 percent, and children, 2 percent.

But he said that some western fire officials in areas he has visited recently estimate that at least 25 percent of their brush and forest fires have been set intentionally. The remains of the incendiary devices left by arsonists are often easy to identify. McCleese described one, but asked that the information not be published so as not to encourage more arsonists