Defence Minister Dr Wayne Mapp has announced he will not be seeking re-election in 2011.
His North Shore seat has been held by National since 1949.
Mapp was first elected to the seat in 1996 and joined the Cabinet in 2008 following John Key's election as Prime Minister.
He also held the portfolio for Science and Innovation.
Dr Mapp said the the death of six military personnel, including three on board a military helicopter that crashed near Wellington and a Kiwi soldier serving in Afghanistan in the British Army, had made it a difficult year.
However, he was pleased with the release of a Defence White Paper designed to meet New Zealand's security needs for the next 20 years.
Dr Mapp said he was 59 and it was time to go.
He expected a hot contest to pick the new North Shore candidate.
"It's a wonderful opportunity," he said.
Dr Mapp, who is married to top lawyer Denese Henare, hadn't made a decision about his future outside Parliament.
Before entering politics he was an associate professor in commercial law at Auckland University and served as a captain in the Royal New Zealand Territorial Army.
Is the tide turning for David Cunliffe?Related story: Cunliffe: 'I'm going to let people in'