A man and woman were seriously injured in a two-car collision near Steyning last night.
The collision happened on the A283 at 10.30pm.
A blue Seat Leon travelling north was in collision with a black Vauxhall Corsa travelling south close to Shoreham Flyover, police said.
The front seat passenger of the Leon – a middle-aged woman – and the front seat passenger of the Corsa – a man believed to be in his teens or early 20s – sustained serious injuries.
They were taken to Royal Sussex County Hospital in Brighton for treatment.
The drivers of both respective vehicles – a man and a woman – sustained minor injuries, police added.
A Sussex Police spokesman said: "Police are urgently seeking the driver of a third vehicle which overtook the Leon heading towards Steyning, and which may have been involved in the incident. The vehicle did not stop at the scene.
"Anyone with any information is asked to report it online or phone 101, quoting Operation Broyle."
The road was closed in both directions until 3am this morning.