High mountain passes and blistering temperatures have taken their toll of vintage automobiles participating in the $250,00 Great American Race, organizers said Saturday.

The sixth annual 4,500-mile road rally, sponsored by Interstate Batteries of Dallas and limited to vehicles dating from 1935 and before, began Wednesday at Disneyland in Anaheim, Calif., and concludes July 3 in Boston.Twenty of the drivers who began the race had dropped out by Saturday, most succumbing to the rigors of steep grades in the Sierra Nevada and temperatures that soared well over 100 degrees in Lancaster, Calif., Sacramento, Calif., Elko, Nev. and in Utah's western desert, said chief photographer Tom Kelsey.

The racers are "car enthusiasts traveling across country to test their endurance and concentration" for up to 15 hours a day, he said.

In each day's leg, drivers travel on interstate freeways to reach side roads or state highways where competition actually occurs. The drivers are clocked by computer and each day's winner is awarded part of the purse. The final winner in Boston claims $50,000, Kelsey said.

Saturday's leg was from Elko to Rock Springs, Wyo.