A bus overturned near a busy intersection in Bethesda yesterday, and nearly two dozen young people lay inside trapped and bleeding, some of them dying fast.

Or at least you'd have thought so.

But awaiting the arrival of the Bethesda-Chevy Chase Rescue Squad and the Bethesda Fire Department, a victim would occasionally smile or chat with someone beside him in the carnage. And nearby, the rescue squad's Ladies Mobile Unit served hot dogs and soft drinks to onlookers.

A drill, of course. And a splendid day for one -- a sparkling sky, a soft breeze. A fine afternoon to be outside.

"It's very easy to sit in classrooms all day and see slides and movies of a wreck," said Rich Adams, spokesman for the 125-member volunteer rescue squad. "But until you see one, work on one, you really don't know how to handle it. And that's the purpose of this."

Rescue squad members -- who had been told that a mock disaster was planned, but not where or what kind -- waited at their headquarters about a half mile from an empty lot near Old Georgetown Road and Wisconsin Avenue, where an old bus donated by a junk dealer had been turned on its side.

An engine company of the Bethesda Fire Department -- knowing a drill was in the works, but nothing more -- stood by at its station about a half mile in the opposite direction.

At the lot, two Red Cross workers smeared red, black and blue greasepaint on the chests, faces, arms and legs of mostly teen-age volunteers -- indicating broken ribs, shins and other injuries -- and gave some of them a bloody final touch of red food coloring mixed with starch and water.

"They told us they wanted a certain number of trauma cases and a certain number of second-priority cases," said Larry Yurow, 19, a Red Cross volunteer. He said he fabricates wounds on volunteers at weekly Red Cross first aid classes, and has become adept at it. As he spoke, he sculpted a large welt on the side of a teen-ager's neck.

"We use wax," he said, "and then I'll make a cut in the wax for the wound, and then we'll add in some blood."

Shortly after 1 p.m., with the wounded piled into the bus, an alarm went out. Minutes later, a fire engine, rescue truck and ambulance rolled into the lot, emergency lights flashing.

Seeing the overturned bus, the firefighters and paramedics called for help, and three more ambulances appeared moments later.

Rescuers scaled ladders to the top of the bus, others fanned out around it, and all began hacking, sawing and drilling their way inside. Still others prepared a staging area for the wounded.

In the bus, rescuers knelt among the bleeding, evaluating injuries, deciding who should be removed first, and how. "Triage," Adams said. "What we do is divide them into three groups: the dead, the seriously wounded and the walking wounded." The seriously wounded got immediate attention.

The injured were carried one at time to the staging area, where paramedics waited with bandages, medical instruments and stretchers. More triage. Those in the most critical need of treatment would have been the first to be taken to hospital, if the drill had gone that far.

But it was confined to the empty lot. Assistant Chief Dave Chaconas of the rescue squad, who was evaluating the exercise, said he would view videotapes of the drill before making a final judgment of the rescuers' performance.

As for the victims, they appeared to enjoy themselves. "I'm a burn trauma," said Elissa Wasserman, 20, of Bethesda. "I'm losing a lot of fluid, supposedly, and my hand and shoulder hurt. But I can't feel anything in my arm, because the nerves are all gone.

"That's what they told me, anyway."