When 3 1/2-month-old Jonathan Jarvis stopped breathing and turned blue Sunday afternoon, the Fairfax County teen-ager who was watching him began to shake and sob uncontrollably.

Then Marci Brunson, 15, broke through her panic. Using the cardiopulmonary resuscitation technique she had learned two years earlier in a freshman health class at Langley High School, she breathed life back into the infant's lungs.

Yesterday, Jonathan continued what is expected to be a complete recovery at Fairfax Hospital, and Brunson was showered with praise for her presence of mind.

"My wife Vicki and I are exhilarated and we thank the Lord," Jonathan's father, Charles Jarvis, told reporters at a news conference at Langley.

"We are just thankful to Marci for her grace under pressure . . . . "

Still shaken, Brunson relived the agonizing minutes when she raced against the clock to revive Jonathan. With Jonathan's fraternal twin Christopher perched on her lap yesterday, she urged parents and baby sitters to study pediatric CPR.

"If I did not learn CPR, Jonathan would not be alive right now," she said, echoing the sentiments of Fairfax fire and rescue officials who hoped her story would send an important message. (Information on free CPR lessons is available at 691-4CPR and by calling local fire departments.)

The traumatic episode began as a homework assignment for a child development class. Brunson was required to observe young children, and she thought of the Jarvis family's four boys, whom she knew from church.

The boys' parents left Brunson to spend some time alone with the children at their McLean home, Brunson recalled. She had been playing with Christopher when she went to check Jonathan in his crib. The baby was cold and still.

"I don't know if I could do the same thing ever again, it was just so frightening," Brunson recounted in a quavering voice, her eyes watering. "But I knew that something had to be done. I just couldn't sit there and watch him."

Brunson, who plans to become a pediatric nurse, checked the baby's pulse and found none. She began administering CPR. Jonathan did not respond. At 2:09 p.m., Brunson dialed 911. Fire department officials yesterday played a recording of her conversation with the emergency dispatcher.

At first, Brunson is crying and has difficulty telling the dispatcher that she needs help at 9058 Jeffery Rd. "The baby I'm baby-sitting is dead," she says.

But with the dispatcher's encouragement, she leaves the phone to attempt CPR anew. A couple of minutes later, the teen-ager's voice returns.

"His eyes are open and he's whimpering. He's moving his arms now," she says.

"Good girl, good, you did a good job," the dispatcher exclaims.

Rescue workers arrived at the house at 2:15 and took Jonathan to the hospital, where he underwent a battery of tests yesterday.

Charles Jarvis said the infant was "doing very well" and was expected to leave the hospital today or tomorrow, although doctors do not know what caused his heart to stop. For a few months, Jarvis said, the twins will sleep with heart monitors.

Jarvis said he and his wife were going to take a course in pediatric CPR last night.