Electrocardiograms taken from 11 patients in sinus rhythm with ventricular ectopic rhythms from two different foci were analyzed to find the number of sinus beats, S, between the ectopic rhythms (S values). Three out of 11 patients had the S values typical for concealed ectopic rhythms. One of them had concealed bigeminy of 2n-1 form that occasionally shifted to 2n form. Following the shift, S values of 2n-1 form were always achieved by the occurrence of double ventricular ectopic rhythms in succession. Concealed trigeminy of 3n and 3n-2 form was seen in the other two patients. Double ventricular ectopic rhythms had bizarre abnormal QRS complexes of two different morphologies and were inscribed in opposite directions. Ectopic rhythms in each case had parasystolic characteristics. These observations suggest bifocal automaticity as a mechanism for bidirectional ventricular tachycardia.