A 16-year-old Fairfax County high school athlete collapsed and died yesterday during the first day of football practice at Robinson High School.

Jon Walsh, described by school officials as a "fine physical specimen, one of the best at the school," had just finished a 40-yard dash and was two-thirds through a mile-and-a-half run on the school's track when he collapsed shortly after 11 a.m.

Within seconds, according to Fairfax fire and rescue spokeswoman Stephanie Strass, Robinson football coach Ed Henry and his staff were at Walsh's side administering cardio-pulmonary resuscitation; rescue medics arrived five minutes later. But efforts to revive the youth failed, and Walsh, who would have been 17 in another two weeks, was pronounced dead at 2:45 p.m. at Commonwealth Doctors Hospital.

The cause of death will not be announced until an autopsy is completed. Sports medicine expert Dr. Gabe Mirkin, queried by a reporter, said he believes that "the odds are 99 out of 100 it was a heat stroke."

Last year 13 high school athletes died as a result of participation in football -- nine from injuries that resulted from physical contact and four as a result of such illnesses as heat stroke or heart attack, according to a survey by the American Football Coaches Association. More than 1.3 million youths played high school football during that year, according to the National Federation for State High Schools.

The football program in Northern Virginia high schools is generally seen as among the best organized and most carefully conducted in the Washington area. Henry, the Robinson football coach, is highly regarded

Walsh was the son of Navy Cmdr. Robert E. and Carol Walsh of 10120 Dundalk St. in the Kings Park West section of Springfield, a few blocks from the sprawling Robinson school, where the youth would have been a senior this September.

"He was a super kid," the senior Walsh said. "The Lord must have taken the best first."

Stunned and teary-eyed, Jon Walsh's friends gathered at the family home, exchanging memories and looking through photographs chronicling the young man's athletic accomplishments. In one picture, Walsh is being honored for placing sixth in the state heavyweight wrestling championship competition.

"He was a super, all-around athlete," said classmate Damon Kuzemka. "He was always working out, lifting weights."

Walsh, who was about 6 feet tall and weighed about 190 pounds, had just returned from a month-long wrestling camp at Iowa State University, where he worked out every day, according to friends.

Chuck Heaton, a friend and fellow football player, said Walsh had gone to bed early Sunday night after attending a dinner for last year's lettermen at Coach Henry's home. "He had just had a physical on Sunday," Heaton said, "and he had a very rigorous physical before he went to wrestling camp."

Laura Guthrie, 15, who dated Walsh since last year when they met at a Kings Park West swimming pool, said, "He was Mr. Macho, Mr. Number One . . . If you got to know him, he was adorable. He was special."

Guthrie was supposed to meet Walsh after football practice. She heard the news while she was working as a lifeguard at Kings Ridge Pool in the subdivision.

Walsh had played offensive guard on last year's Robinson team, and this year was trying out for center.

He was the third of six children in the Walsh family. The oldest child, Michael, graduated from Robinson in June and Cathy, the second oldest, will be a sophomore at West Point this fall.

When Walsh collapsed during yesterday morning's inaugural practice, the temperature was about 85 degrees, the humidity about 57 percent and there was a 5-mile-an-hour wind. According to sports medicine expert Mirkin, "those conditions were not that bad." But he stressed that even a physically fit athelete could suffer a heat stroke in those conditions "if he was not acclimated to exercising in the heat."

Mirkin, who himself suffered a heat stroke during a marathon race in the 1960s, said athletes exercising in hot weather should drink a cup of water 15 minutes before any workout and periodically during the workout. He also said it takes about 14 days to become acclimated to hot-weather exercise.