The time course of the ventricular fibrillation threshold (VFT) was studied in 10 open-chest mongrel dogs following acute occlusion of the left anterior descanding coronary artery (LAD). The VFT in the infarcted area was compared to a non-infarcted area of the left ventricle supplied by the circumflex coronary artery. Prior to coronary occlusion the mean VFT for the entire group of 10 animals was 14 ma. for the area supplied by the LAD and 17.6 ma. for the area of the left circumflex. Immediately after coronary ligation the VFT decreased in both areas. Within 90 to 120 minutes the VFT in the infarcted area was 27 ma. and after 6 hours of occlusion the VFT was 3 times the pre-ligation value. The VFT in the non-infarcted area remained near the pre-ligation values. The excitability of the infarcted area was markedly decreased after coronary occlusion and this accounted for the increase in the VFT in the infarcted area. In the non-ischemic area the excitability was unchanged during the entire six hour period following occlusion. The study stresses the importance of the location of the electrodes used for fibrillation and the natural course of the VFT in evaluating VFT's within the ischemic myocardium.