A woman in her 20s sustained life-threatening injuries when the car she was in crashed into a tree in Maidstone.
It happened on the A249 Sittingbourne Road, near the junction with Sportsfield, just after midnight.
A 25-year-old woman from Sevenoaks was arrested in connection with the crash and is being questioned by police.
The road was closed for more than seven hours while accident investigators examined the scene.
Kent Police said the blue Peugeot convertible was heading towards the M20 when it left the road.
The force is keen to speak to witnesses and a man who was reported to be in the car before the crash.