Abstract: A device for terminating tachycardia emits stimulation pulses to the heart synchronized with the heartbeats and a variable time delay. In order to quickly determine the proper delay time and thus increase the chances for successfully terminating tachycardia, the device has a unit for measuring the time between successive heartbeats and for deriving a plurality of different delay times therefrom of which the shortest delay time is a selected initial value and the difference between successive delay times becomes increasingly greater. The delay times are stored in a register which may be scanned in a specific sequence.