The behavior of right and left ventricular endocardial potentials (ECP) was studied in open-chest anesthetized dogs, both before and after ligation of the coronary artery. Endocardial potentials were recorded from the anterior and inferior walls of the left ventricle (LVECP Ant., LVECP Inf.) and from the inferior wall of the right ventricle (RVECP). Direct epicardial potentials from the anterior and inferior surfaces of the left ventricle, along with ECG Lead II, were also recorded. LV dp dt and aortic pressure were monitored simultaneously. Following ligation of the coronary arteries, LVECP Ant. fell in all animals, and LVECP Inf. and RVECP in most, and remained lower than the control values throughout the periods of observation. Fall in ECP occurred whether the endocardial electrodes were in contact with ischemic or nonischemic endocardium. No relationship was found between the postocclusion changes in simultaneously recorded epi- and endocardial potentials whether from the anterior or inferior surface of the left ventricle. Neither was any correlation between the changes in endocardial potentials recorded from anterior and inferior surfaces of the left ventricle or between LVECP and RVECP. These findings suggest that the fall in ECP which occurs following acute coronary artery occlusion is unlikely to be due to the phenomenon of cancellation between electrical forces generated at different parts of the ventricle. Neither localized ischemia nor infarction of the myocardium seems directly responsible for the fall in ECP. The precise mechanism remains obscure. Fall in LV dp dt and in aortic pressure occurred in most animals following ligation of coronary arteries and there was moderate correlation between postocclusion changes in LV dp dt and changes in ECP.