A woman was discovered with serious head and facial injuries in the early hours of yesterday morning (Sunday) in Wellingborough.
Police investigating how she suffered the injuries are appealing to trace the occupants of two cars which passed her.
Between 2.20am and 3am the woman was found after being seen walking unsteadily in Thomas Street.
Enquiries showed two cars had driven along the street at around the same time, one white in colour, the other dark.
A police spokesman said: "In order to help establish how the woman came to be hurt, officers would like to speak to the drivers of these vehicles and any passengers they may have had about what they may have seen.
"Anyone with information about this incident is asked to call Northamptonshire Police on 101, or Crimestoppers in confidence on 0800 555111."