Willie and Gerry Garrett, who live with their two teen-age sons in southern Prince George's County, were alarmed when police awakened them "at 12:37" yesterday morning.

"They said that there were poisonous fumes in the area and we had to leave," Gerry Garrett said yesterday afternoon. "The red lights were flashing. It was frightening."

The Garretts joined about 100 other people who were forced from their homes in the Cheltenham area of the county after a tanker truck overturned and spilled toxic oil nearby. By noon yesterday, the Garretts had left the emergency shelter where they spent the night and police had reopened a section of Rte. 301 that had been closed for 15 hours.

Meanwhile, at 5 a.m. in Montgomery County, a tanker truck also overturned and spilled oil on River Road near the Capital Beltway. That accident snarled the morning rush hour, but traffic was back to normal by midmorning, police said.

Prince George's County Fire Department spokesman Anthony DeStefano said a tanker truck carrying 4,100 gallons of aniline oil overturned at 9 p.m. Thursday near Rte. 301 and Frank Tippet Road, leaving the driver and a passenger pinned in the cab. Six firefighters at the scene were sent to Southern Maryland Hospital and treated for exposure to toxic fumes. They were released after observation, the spokesman said.

The oil, a poisonous substance that is used to make Styrofoam plastic and can be fatal if inhaled or if it touches the skin, was leaking from the tanker at a rate of 15 to 20 gallons an hour, DeStefano said.

County and state police warned residents in two subdivisions, Cheltenham Woods and Terraco Acres, of the danger and directed them to an emergency shelter at the Mattaponi Elementary School in Upper Marlboro. DeStefano said the county's hazardous materials response team had determined that workers needed a 3,000-foot buffer zone while they worked to stop the leak and transfer the oil into another tanker.

Maryland State Police said the accident occurred when a car driven by Louis John Grimaldi of Cheltenham crossed Rte. 301 at Frank Tippett Road and was struck by the tanker truck driven by John Please McClain of Beaumont, Tex.

Grimaldi, McClain and a passenger in the truck, Harry Gilder, also of Beaumont, were taken to Southern Maryland Hospital.

Grimaldi and Gilder were in serious condition in the intensive care unit and McClain was in stable condition, according to a hospital spokesman.

Grimaldi, who was not wearing a seat belt, was charged with failure to obey a traffic control device, state police said. McClain was wearing a seat belt, state police said, but his passenger was not.

The accident in Montgomery County occurred when a taxicab driven by Robert William Cramer pulled onto River Road from the Holton-Arms School in North Bethesda, said Cpl. Phil Caswell, a Montgomery County police spokesman.

The taxi was hit by a home heating oil truck driven by Patrick Blair Miller, police said. The truck overturned and began to leak oil, police said.

Miller, 26, of 10205 Sunderland Rd., Silver Spring, was taken to Suburban Hospital where a spokesman said he was in critical condition in the intensive care unit. Cramer was not injured.

Cramer, 31, of 8629 Piney Branch Rd., Silver Spring, was charged with failure to yield the right of way, police said. Both drivers were wearing seat belts, police said.