The final stop for Johnson

''I'd love to still be playing. If I could play 10 more years I would. If my body would let me I would."

Johnson said he went for a general physical with his personal physician, who said there was enough evidence for Johnson to have serious concerns that his symptoms -- irritability, difficulty sleeping, and memory loss -- were the result of previous head injuries.

Specialists recommended he step away from the game.

''It was strongly urged and suggested that I not play; that I should consider the ramifications," Johnson said. ''I can still play, but I open myself up to potentially some very damaging long-term health issues.

''When it's time to go, you better go. When your body tells you it's time to go, you'd better listen to it."

Though he couldn't remember the number of diagnosed concussions he has suffered playing football, Johnson estimated a half-dozen, with likely many more that went undiagnosed. After lingering symptoms in the fall of 2002, he underwent neurological testing at Massachusetts General Hospital, but continued to play.

He admitted that perhaps he played some of the 2004 season with concussion symptoms, though he started 15 of the Patriots' 16 regular-season games and finished third on the team in tackles with 112.

Johnson pointed to former Dallas quarterback Troy Aikman, who retired in 2001 at age 34 after 10 concussions, as part of a history of head injuries that NFL players need to pay attention to.

''We didn't know much about [concussions], and football players, we don't really want to know," Johnson said. ''There were times I'd second-guess my decision to play, but I loved the game. I was playing for all the pure reasons."

Johnson's exit comes eight days after fellow inside linebacker Tedy Bruschi decided to sit out the 2005 season as he recovers from a stroke suffered in the offseason. Teammates for the better part of a decade, they first started together on the same defense in 1997.

Johnson said if Bruschi's health issues had anything to do with his situation, it was purely on a subconscious level, as Johnson's problems have been ongoing for a few years.