Abstract: A process and an apparatus for producing hollow bodies of thermoplastic material by a blow molding procedure use a blow molding mold comprising at least two portions, with at least one means by which the mold portions are urged towards each other in the mold-closing direction and are locked together when the mold is closed. The locking means carries a large part of the forces resulting from the blow molding pressure and possibly also at least a part of the forces required for squeezing surplus material off the preform to be expanded in the mold. The forces applied by the locking means are transmitted to the mold portions, with the interposition of at least two spaced-apart hydraulic force-transmitting means. The hydraulic force-transmitting means are hydraulically connected together in order to render uniform the forces which act on the mold portions.