A motorcyclist and his female passenger were airlifted to hospital with serious injuries after a road traffic collision in Woburn Sands.
They were both taken to the John Radcliffe Hospital where the woman, 30, remains in a critical but stable condition.
The collision happened at 1.25pm in Bow Brickhill Road between Bow Brickhill and Woburn Sands and involved a black and red Kawasaki motorcycle and a black BMW car.
The driver of the BMW was uninjured.
PC Ian Stevens of the Milton Keynes Road Policing Team said: “I would urge anyone with information to come forward and speak to me as soon as possible.
“I am keen to speak to any witnesses who saw either vehicle before or at the time of the collision.”
After the collision, which happened at the junction of Bow Brickhill Road and Woburn Sands Road, both roads were closed.
Any witnesses can call police on 101 or Crimestoppers, anonymously, on 0800 555 111.