Using a three part model of canine false tendon in which an inexcitable gap intervenes between the proximal and distal region of Purkinje fibers, Jalife and Moe recently individualized a biological model of parasystole and showed how the electrotonic depolarization can modulate, entrain or annihilate pacemaker activity. We report the ECG of 9 patients with parasystole (as assessed by the direct recording of ectopic cycle length) in which the early nonparasystolic beats delayed whereas the late shortened the parasystolic cycle length. The phase response curve is specific for a given patient. In two cases a nonparasystolic impulse falling at a critical point in the ectopic cycle length was followed by the suppression of any parasystolic activity on the surface EKG. The recently described phenomenon called pace maker annihilation is thus discussed.