John Yeardley, an NBC radio engineer on the late shift, reached his Montgomery County home in the early morning darkness yesterday to find his wife lying unconscious on the garage floor and their younger daughter lying nearby.

Yeardley's wife, Marie, 39, was revived at Montgomery General Hospital. Their daughter, Felicia, 11, who loved animals and took art lessons, died at the hospital shortly before 5 a.m.

Both had been listening to tapes in one of the family cars during the night, and both had apparently been overcome by fumes. Police said it was an accident.

"I'm kind of in shock still," Yeardley said last night, "It's not something you expect to find when you come home from work."

It was about 11 p.m. Friday night when Felicia went into the enclosed garage that is built into the family's two story brick house at 17220 Georgia Ave., near Olney.

There was no cassette tape player in the house, but there was in one of the cars. Felicia, a fifth grader at Calvary Luthern School in Montgomery Hills, began listening to tapes in the car and after a time, her mother joined her. The older Yeardley daughter, Valerie, a sixth grader at the Lutheran school, had gone to bed earlier.

While mother and daughter sat in the car, Mrs. Yeardley, according to a police account, turned on the car's engine to keep the battery from going dead.

About 2 a.m., according to the same account, they started to feel drowsy, and got out of the car. Mrs. Yeardley collapsed near the car. Felicia was overcome in a laundry room adjoining the garage.

Yeardley, assigned to a 5 p.m. to 1 a.m. shift at WRC and WKYS radio stations here, worked a little later than usual Friday night, he said, and came home about 2 a.m.

The car in the garage was not running when he arrived. The key, according to one source, was found near where Mrs. Yeardley was lying.

Mrs. Yeardley was reported in satisfactory condition in the intensive care unit at Montgomery General last night.

The cause of Felicia's death, has not been determined officially, but authorities said she was apparently asphyxiated by carbon monoxide fumes that were generated by the car that accumulated in the closed garage. An autopsy is scheduled.

Although police attributed the death to fumes from the car, Yeardley told a reporter last night that he thought it possible that the car engine had not been turned on and that fumes may have come from a newly installed oil burner.

According to Yeardley, his daughter kept her room full of pets, including mice, gerbils, hamsters, cats and dogs and hoped to operate a pet store someday.