Two 15-year-old boys on a stolen moped going the wrong way on a one-way street and being followed by police were injured last night when the moped slammed into a pickup at Russell Road and West Glendale Avenue in Alexandria, police said.

One of the youths was reported in serious condition at Inova Fairfax Hospital. The other, at Inova Alexandria Hospital, was expected to be released, officials said.

Two police cruisers were following the moped, which had been reported stolen two days ago. The collision occurred about 8:20 p.m., police spokeswoman Amy Bertsch said. She said the moped was going east--the wrong way--on one-way Glendale.

The moped hit the side of a Toyota pickup truck at the intersection, Bertsch said. She said the police cars were not involved in the collision.

Bertsch declined to talk about the speed of the moped or police and the distance between the vehicles. "It's still under investigation," she said.

Matthew Gemmell, 28, who said he was dining with his parents in the neighborhood, said he saw the police cars, with lights flashing, about four car lengths behind the moped when the crash occurred. He estimated the moped's speed at 20 to 25 mph.

"They were following the guy; they did not push this guy into the intersection," Gemmell said.

He said he saw both boys thrown from the moped. One lay face down in a gutter; the other was walking soon after the accident, Gemmell said.

"I was amazed that one of the kids got up," he said. "They hit the truck with such force."