Abstract: For stretching blowing up a preheated preform, the preform is mechanically stretched longitudinally by a rod simultaneously to its blowing up, causing its transverse deformation. The displacement of the stretching rod is caused by a pneumatic piston fed by the same pressurized air supply as feeds the inside of the preform for its blowing up. Air is supplied under pressure inside the preform through a regulating valve which does not open until the stretching rod has contacted a closed end of the preform. An open end of the preform is sealed against a sleeve which is mounted for movement relative to but in the same direction as the rod. The pressurized air is applied against this sleeve to force the sleeve into sealing contact with the open end of the preform with a sealing pressure that varies as the pressure of the pressurized air. A piston on the sleeve rides in a cylinder into which the pressurized air is introduced on a side of the piston opposite the preform.