Ventricular tachycardia and fibrillation have been induced in the intact ventricle of the guinea pig by the use of aconitine. The micropuncture technique was used to record single fiber electrical activity. Ventricular fibrillation was characterized by irregular action potentials of varying amplitude. Simultaneous recording of electrical activity from two fibers on the ventricular surface revealed partial synchronism of action potentials when the fibers were in close proximity. The synchronism was not evident when the explored fibers were at a distance. The disorganization of guinea pig ventricular fibers during fibrillation was not total. Single fiber electrical alternans was frequently observed during ventricular tachycardia and was associated with mechanical alternans. Mechanical alternans was seen occasionally without demonstrable single fiber electrical alternans. It is suggested that in this circumstance the appropriate fibers were not selected for microelectrode penetration, since it was found that all cells did not participate in electrical alternation in any one preparation. It would appear, therefore, that alternation of the heart, both electrical and mechanical, may be related to alternate variations in behavior of individual fiber membrane and contractile elements.