A.C. stimuli (60 cycle) composed of 1/2 to 4 waves (0.008-0.0666 sec.), like brief D.C. shocks, act as a unitary stimulus. When they start or fall entirely within the vulnerable period, they always give 1 or 2 premature ventricular systoles followed or not by ventricular fibrillation. A similar stimulus during the refractory period is ineffective. When given during diastole, it causes one premature beat and never produces fibrillation. Stimuli composed of 7 to 9 sine waves produce no effect or an occasional premature beat when they are very weak. A similar number, of moderate strength, falling entirely in the refractory phase are ineffective; but when they encroach upon, cover, or start in the vulnerable period, they evoke a single premature contraction, but never cause fibrillation. If they start ever so early in diastole, however, fibrillation occurs. Strong currents also cause fibrillation when they enter during the vulnerable period. The occurrence of fibrillation by early diastolic shocks, incapable of acting during the vulnerable period, can be explained by the fact that an effective portion of the series now falls during the vulnerable period of a premature beat. In other words, an apparent diastolic fibrillation is actually a fibrillation started during the systolic vulnerable period of a premature beat. Even very weak A.C. stimuli, with durations varying from 0.2 to 1 sec. or more cause ventricular tachycardia which may revert to a normal rhythm after removal of the stimulus or may eventuate in fibrillation.