A woman was taken to hospital after she was discovered lying unconscious in Avenham Park, say police.
Police were called out by a member of the public who discovered the 24-year-old by the river just before 9pm on Tuesday evening.
Officers delivered first aid until the arrival of paramedics.
A spokesman for Lancs ARV said in a Tweet: "Car tasked to Preston today has just finished using enhanced first aid on an unconscious lady by the river in Avenham Park.
"Quickly assisted by IR colleagues and @nwas. Lady was given treatment at the scene and now hospital."
Police later confirmed that the woman regained consciousness in hospital.
A spokesman said: "We were called by a member of the public to the report that a woman had been found unconscious close to a bridge on Avenham Park.
"She was taken to Royal Preston Hospital where she regained consciousness..
"It is believed that the woman was out running and could have suffered a medical episode and/ or fallen over.
"There is nothing to suggest she was subject to anything suspicious at this time."