Ray Nitschke, one of the most fearsome middle linebackers in the history of professional football and an anchor of the championship Green Bay Packer teams of the 1960's, died yesterday in Venice, Fla. He was 61.

Nitschke, who had a winter home in Naples, Fla., suffered a heart attack while driving to the house of a family friend, said his daughter, Amy Klaas, who was with him when he was stricken. He was pronounced dead at Venice Hospital.

The personification of the rough-and-tumble linebacker who could smother a running back and level a quarterback with equal aplomb, Nitschke was elected to the Pro Football Hall of Fame in 1978 and was selected for the National Football League's 50th and 75th anniversary all-star teams.

His old coach Vince Lombardi once called pro football ''a game that requires the constant conjuring of animosity.'' Nitschke was not huge -- he stood 6 feet 3 inches and weighed 235 pounds -- but he fit Lombardi's mold perfectly, once remarking: ''My father died when I was 3, my mother when I was 14, so I took it out on all the kids in the neighborhood. What I like about this game is the contact, the man-to-man, the getting-it-out-of-your-system.''

Playing for the Packers from 1958 to 1972, Nitschke teamed up with fellow Hall of Famers such as Bart Starr, Paul Hornung, Jim Taylor and Jerry Kramer, winning five league championships and starring on the teams that won the first two Super Bowls, in 1967 and 1968.

Nitschke was an unheralded collegian, having been drafted in the third round after playing fullback at the University of Illinois. But he was named to All-Pro teams in 1964, 1965 and 1966 and was voted most valuable player in the Packers' 16-7 victory over the Giants at Yankee Stadium in the 1962 N.F.L. championship game. On a windy day, with the temperature 10 degrees at kickoff, Nitschke recovered two fumbles and deflected a pass by the Giant quarterback, Y. A. Tittle, enabling another linebacker, Dan Currie, to intercept it.

In a recent poll conducted by The Milwaukee Journal-Sentinel among former Packer players and coaches, he was voted the fourth-best player in Packer history, behind receiver Don Hutson; the current quarterback, Brett Favre, and Starr, the quarterback on Lombardi's teams.

Starr once said that Nitschke was strong enough ''to throw a ball 100 yards at times, and 80 yards like it was nothing.'' A running back named Jim Race was once knocked unconscious for 15 minutes after being tackled by Nitschke.

That title game with the Giants was frigid enough, but Nitschke would remember a game five years later for the most difficult conditions he ever played in. That was the matchup known as the Ice Bowl, when the Packers played the Dallas Cowboys at Lambeau Field in Green Bay on New Year's Eve 1967 for the right to advance to Super Bowl II. The temperature at game time was 13 below, but the teams put on an exciting show into the final seconds, when Starr scored on a quarterback sneak to give the Packers a 21-17 victory.

''The Ice Bowl was the culmination of the Lombardi era in Green Bay,'' Nitschke remembered some 30 years later. ''We were going for our third championship. The Cowboys were an up-and-coming team approaching greatness. But you always took the field hearing Lombardi's voice in your mind. He taught us how to win under any conditions. He taught us you're always in the game. He told us it's 60 minutes long.

''But as the game went on, the field got icier and icier and all my toes blistered up. I didn't even think about my feet. I didn't realize until a couple of hours after the game that I had frostbite.

''But the ones I felt sorry for were the fans. Coming out of the tunnel onto the field, I looked up and saw the stadium was filled. I thought, 'If they came out in this weather, I'll give it my best shot.' ''

In addition to his daughter, Nitschke is survived by two sons, John and Richard, and a granddaughter. His wife, Jackie, died in 1996.

Nitschke, who made his permanent home in Green Bay, often attended Packer practices, traveled to many road games and chatted amiably with fans who approached him for autographs, in contrast to his fierce presence on the field. But the fire within still burned.

When the Packers were making a run toward their Super Bowl championship of 1997, a defensive end, Sean Jones, thought back to the stars of the Green Bay teams of the 1960's.

''What people don't understand outside Green Bay is that here we have to exorcise those ghosts -- Willie Wood, Willie Davis, Bart Starr, Ray Nitschke,'' he said. ''I think Ray Nitschke thinks we stink.''

Nitschke said he did not feel that way, but he made certain that his successors played with the intensity he had.

A few days before the Packers won the 1997 Super Bowl, the star defensive end Reggie White remarked on that.

''Ray Nitschke, Willie Davis, Bart Starr, they come around,'' White said. ''Nitschke, he's really intense. He'll come over and smack you in the face and say, 'You can't lose this game.' He's like a coach, but after a while you say, 'Ray, you got to stop smacking me.' ''

Raymond Nitschke (1936)