The sequence of localized changes in ventricular repolarization time during and after temporary coronary artery occlusion was studied in 10 open chest dogs. Immediately after the onset of coronary occlusion functional refractory periods (FRPs) prolonged slightly in the ischemic area, then shortened with continued occlusion. Within the first minute following release of occlusion, FRPs underwent a further brief decrease in duration. By varying the period of occlusion from 1 1/2 to 6 1/2 min, evidence was obtained that the post-release shortening of RFPs was temporally related to release of the clamp and not to the onset of occlusion. Ventricular fibrillation occurred in 2 dogs, in each instance soon after release of the coronary artery occlusion. The possible relationship of these experimental FRP changes to waveform abnormalities and arrhythmias in ischemic heart disease is discussed.