The invention relates to a device (101) for varying the control times of gas exchange valves of an internal combustion engine, with a hydraulic actuation device (102) and with a control valve (103). The device (101) according to the invention is provided with mid-position locking of a hydraulic actuation device (102). Furthermore, the device (101) according to the invention ensures that, in the event of the failure of an actuation unit (112) which regulates the control valve (103), the hydraulic actuation device (102) is locked in the mid-position and the lock is maintained until the actuation unit (112) is repaired. Furthermore, the device (101) according to the invention makes it possible to start the internal combustion engine in a position locked in a mid-position, without a movable element (105) of the hydraulic actuation device (102) butting against a side wall of a pressure space (104) when the internal combustion engine is started. Methods are proposed to bring the actuation device (102) into a locked mid-position, and to hold it there, for the restarting of the internal combustion engine.