Many previous investigators have presented data suggesting that the processes involved in excitation and recovery in heart muscle limit the “vulnerable period” to a relatively small part of the heart cycle in late systole. Evidence presented in this report indicates that there are exceptions to this rule, at least in the intact animal, since under some conditions multiple extrasystoles and ventricular fibrillation can be elicited by single stimuli of short duration and high voltage applied at almost any time in systole or diastole.