SA police find kidnapped man in ute

Four people have been arrested after a man was found with severe head injuries in the back of a ute in Adelaide's south.

A 34-year-old Seaford man and a 28-year-old woman from Western Australia were charged with kidnapping and aggravated assault after police made the discovery early on Tuesday morning.

Police stopped the ute on Commercial Road, Seaford, just after 2am (CST) and searched the vehicle after speaking to the pair.

They discovered a man, aged in his 20s, with severe head injuries on the tray of the ute.

Police allege he was assaulted at several locations across the southern suburbs.

The man, of Morphett Vale, is being treated in hospital.

Police later charged a 36-year-old Seaford Rise man with aggravated kidnapping and a 19-year-old man, also of Seaford Rise, with aggravated kidnapping and aggravated possession of a prescribed firearm.

The four people charged were refused bail and are expected to appear in the Christies Beach Magistrates Court on Wednesday.