It is now definitely established from recorded observations that transient syncopal attacks occurring in patients with permanent auriculoventricular dissociation are due in the main to one of two conditions; either to complete standstill of the ventricles or to the various grades of acceleration leading to ventricular fibrillation. The former mechanism, which is the more common, has been excellently reproduced in the dog, in which heart block had been established,1 and from correlations with the clinical and graphic manifestations observed in man there is now a fairly accurate knowledge of some of the successive events leading to and following transient ventricular standstill. Comparable analyses of the disturbances leading to the onset and recovery from transient ventricular fibrillation in the presence of auriculoventricular dissociation are lacking, for spontaneous recovery from ventricular fibrillation in the larger intact animal is unknown if the rhythm is induced by any of the methods now available.