During electrolytical treatment, a printing cylinder is held between center points, of which one is provided adjustably with a centering sleeve in a tail spindle. A belt connects the tail spindle in force transmitting relationship to a drive. The tail spindle is axially displaceable together with the inner housing of the apparatus, so that an electric contact ring at the end of the tail spindle is engageable with the front surface of a printing cylinder. The current conduction to the contact ring takes place via cooled carbon brushes located between the tail spindle bearings and the contact ring. A current conducting sleeve encompassing the end region of the tail spindle and being contacted by the carbon brushes, prevents the tail spindle carrying the printing cylinder from being exposed to electric current.