Two northbound cars collided on the Capital Beltway in McLean yesterday, locked together and flew into oncoming traffic to set off a chain-reaction accident that involved seven cars and one truck and blocked all southbound lanes just as the evening rush hour was beginning.

The accident, between the Dulles Toll Road and the Georgetown Pike exits, was the second major tie-up on the Beltway in as many days and backed up evening rush-hour traffic for more than three miles into Maryland, authorities said.

Four persons were injured in the 3:52 p.m. crash, which began when a blue station wagon swerved from the far right northbound lane into a guardrail, and then rebounded into a Subaru station wagon, Virginia state police and witnesses said.

The two cars locked bumpers, crossed two lanes of traffic, drove up a grassy embankment and became airborne, flying more than five feet above ground before landing on a southbound green Oldsmobile, the witnesses said.

Kriste Lynn Brown of 8930 Arley Dr., Springfield, who was in the Oldsmobile, was flown by helicopter to Fairfax Hospital, where she was reported in critical condition, according to hospital officials. Howard Longhorn of Beltsville, driver of the blue station wagon, was taken by ambulance to the hospital and was reported in fair condition.

Two other persons involved in the pileup were treated for minor injuries at the scene, said Virginia state police Sgt. J.W. Rowles.

"I heard the impact and the next thing I knew we were in the air," said Ashley Stringer, 24, the driver of the Subaru. "It was like tag-team wrestling."

"It was just mayhem," said Beverly Toney, the driver of a UPS tractor-trailer that was caught in the pileup.

"All of a sudden you see cars flying, they came from out of nowhere," said Toney, who said he slammed into a Volvo after hitting his brakes.

Police investigators accompanied the injured to the hospital and were planning to interview them if possible, Sgt. Rowles said. An officer would decide at the hospital whether to require testing for drugs or alcohol and whether to file charges, he said.

Three of the Beltway's four southbound lanes were opened by 4:50 p.m. and the fourth was opened before 7 p.m., but traffic was jammed across the American Legion Bridge over the Potomac River almost to Old Georgetown Road in Maryland, according to Virginia and Maryland officials.

Traffic was slowed on the northbound side by rubberneckers, police said.

Rescue efforts were hampered by the attempts of some southbound drivers to turn around and drive north to escape the accident site and exit at Georgetown Pike, police said.

Rush-hour traffic elsewhere in Northern Virginia was largely unaffected, Rowles said.

For the second day in a row, Virginia Department of Transportation officials immediately notified radio and television stations of an accident and warned commuters to avoid what was certain to be a major tie-up on the Washington area's Main Street.

The department took similar action early Tuesday when a three-truck pileup on the Woodrow Wilson Bridge closed the Maryland-bound lanes of the Beltway for five hours.

The UPS double-trailer truck "was by no means" at fault, Rowles said.

Nonetheless, the second Beltway accident in as many days involving trucks underscored the frequency of such events on that highway and comes at a time when some safety advocates are calling for increased regulation of truckers.

According to statistics compiled by the Potomac chapter of the American Automobile Association, overall Beltway traffic increased 10 percent from July 1984 to December 1985, but during the same period accidents involving tractor-trailers increased 38 percent.

Tractor-trailers account for less than 3 percent of the vehicles on the Beltway, but were involved in 19 percent of the accidents during that 18-month period, the AAA said.

Deregulation of the trucking industry has increased the number of trucks on the road at a time when overall increases in Beltway traffic have strained the highway's capacity, Reynolds said.