A computer model of cardiac excitation sequences was used to reproduce atrioventricular (AV) reentrant tachycardia (AVRT) and its termination by a single 'on-circuit' extrastimulus. The model simulated activation waves revolving along a one-dimensional circular pathway, the portions of which represented the atrial, AV nodal, His-Purkinje, ventricular, and accessory pathway sections of the tachycardia circuit. The modeled pathway was composed of 289 elements. The model distinguished only the depolarised and resting states of constituent elements, but introduced differential refractoriness and conduction velocity for each element. These values approximated the natural situation established in a patient suffering from AVRT associated with the right bundle branch block. The results of the study suggest that: (A) the usual impression of a regular recovery wave and of a regular excitable window moving uniformly along the macro-reentrant circular path is incorrect; (B) during the tachycardia, islands of repolarized cells appear which are surrounded by tissue that is still refractory; (C) an extrastimulus which captures the island of early repolarized tissue may cause an excitation restricted to a small part of the myocardium but the local refractoriness following such an extrastimulus may be sufficient to terminate the tachycardia.