As shown by Jalife and Moe, brief subliminar depolarizations induced upstreams of the protection zone may predictably modify the discharge from a parasystolic focus. In the biological and mathematical models of parasystole, numerous arrhythmias which do not fulfil the classical criteria of parasystole have been identified, some of them even mimicking a re-entry mechanism. In this paper we report the electrocardiograms of three patients in whom an electrotonic modulation mechanism may be involved. In the first patient the classical pattern of continuous parasystole without exit block was observed throughout daytime. However, between 1 and 5 a.m. non parasystolic beats lengthened or shortened the parasystolic rhythm, depending on their time-related position in the cycle. Later, the usual continuous parasystole pattern reappeared. In the second patient the patent 1:1 entrainment of parasystole by sinus beats resulted in long episodes of ventricular bigeminy with fixed coupling. A sudden slowing down of the cardiac rate disrupting the harmony of the two rhythms and, consequently, the regular entrainment of the ectopic focus, shortened the parasystolic cycle and suppressed bigeminy. In the third patient ventricular stimulation produced a hisian parasystolic rhythm resembling intermittent parasystole on surface ECG. It is concluded that, as suggested by experimental models, many disorders of cardiac rhythm which do not fulfil the criteria of parasystole can be explained in terms of modulated parasystole.