A combination lock wherein the tumblers can be moved to predetermined positions, preparatory to retraction of the locking bolt from extended position, by an electric motor which is controlled by a microprocessor. The microprocessor receives signals from a keyboard or from a disc and can start the motor for the purpose of aligning the normally scrambled tumblers only in response to reception of a predetermined sequence of signals. A manually operated knob can be coupled to a driver for the tumblers in order to change the positions of the tumblers and/or to move the locking bolt between extended and retracted positions independently of the motor. The microprocessor is programmed to start the motor only in response to reception of a predetermined sequence of signals in accordance with a selected combination.