Abstract: Apparatus for welding motor vehicle bodies which have previously been assembled loosely comprises a station for the forming and welding of the body, a conveyor line for transporting the loosely preassembled bodies to the station, at least two pairs of forming frames situated at the two sides of the station, each pair of forming frames being provided with positioning devices engageable with the body and adapted to the specific geometric shape of particular type of body, forming-frame conveyor means for bringing a pair of forming frames corresponding to the type of body which is in the welding station into correspondence with an operative position in the welding station from time to time, and at least one welding device for welding the body. At least one drum which can rotate about an axis parallel to the length of the welding station and carries on its periphery a plurality of forming frames suitable for various types of body is provided on each side of the conveyor line.