Abstract: A device for terminating tachycardia has an input part responsive to heartbeat signals. A tachycardia rate detector is connected to the input part and is effective to give an output while heartbeat signals occur at a rate characteristic of tachycardia. A gate circuit is connected to be controlled by the output from the tachycardia rate detector and a variable delay pulse circuit is connected to receive as input pulses heartbeat signals from the input part and give output pulses each a predetermined delay after a respective input pulse, the output pulses being applied as input to the gate circuit. An output circuit connected to supply stimulating pulses to the heart, the stimulating pulses being generated in response to gated pulses received from the gate circuit. A blocking circuit is connected to the gate circuit to ensure that output pulses from the gate are inhibited during a pause period following each output pulse.