North Melbourne premiership midfielder Shannon Grant will retire at the end of this season, admitting his body can no longer cope with the rigours of AFL football.

The 31-year-old announced his decision yesterday, before his 300th game against Port Adelaide at the MCG on Saturday. Grant, one of the Kangaroos' most decorated players, was the Norm Smith medallist in their 1999 flag-winning side, an All-Australian in 2005 and his club's best and fairest in 2001.

Grant said he wanted to leave while he was still a key member of the senior team but conceded he was having trouble keeping up with the modern game.

He said, ''Two or three months ago the body was starting to not recover as well as it has in the past and it took me a lot longer to get up throughout the week.

''I've never had the quickest legs and they're slowly starting to go as well.

''I've always pretty much wanted to go out on my own terms: I don't think there's anything worse than going that extra year or two and [to] end up running around in the VFL. That's something I didn't want to do.

''I think when you get to this stage of your career you know deep down when the right time is to retire.''

Grant admitted he had been unsure of reaching 300 games: early last month the club suspended him for one game for an off-field indiscretion that was never explained.

Grant acknowledged modern football was ''probably catching up with me''.

He said, ''It's such a strong, quick game these days and you've really got to be on your toes.''

He played 58 games with the Sydney Swans before joining the Kangaroos in 1998. AAP