At around 4.45pm on the Bletchley Road in Thornborough, at the junction with Padbury Road, a red and black Suzuki motorcycle was travelling along the A421 towards Buckingham when it was involved in a crash with a white Mini which pulled out of Padbury Road.
The motorcyclist, aged 42, from Northamptonshire, was airlifted to the John Radcliffe Hospital in Oxford from the scene.
He sustained serious injuries but is currently in a stable condition.
The driver of the Mini, a woman aged 48, and also from Northamptonshire, was taken to Milton Keynes Hospital by ambulance.
Her injuries are minor and she has since been discharged from hospital.
Anyone who witnessed the crash or who may have seen any of the vehicles involved before the collision is asked to call Thames Valley Police on the non emergency number 101 and quote the URN 1006 5/8/15.
Alternatively, contact Crimestoppers anonymously on 0800 555 111.