A woman was airlifted to a Toronto hospital following a head-on collision on Highway 35 south of LIndsay on Thursday afternoon.
The collision occurred around 4:15 p.m., prompting Kawartha Lakes OPP to close the highway in both directions between Highway 7A and Lifford Road, just west of the village of Bethany.
One car ended up in a nearby ditch, while the other was in the centre of the highway.
An Ornge air ambulance landed on the highway to transport one patient.
According to Ornge, a woman in her 50s who was the driver of one vehicle was transported with critical injuries.
On Friday, OPP say three occupants of the second vehicle were transported to hospital with unknown injuries.
At least six ambulances – from Peterborough and the City of Kawartha Lakes – were at the scene.
The highway re-opened around 7 p.m.
OPP said the investigation is ongoing.