Because the automatic implantable defibrillator may be used in patients having ischemic heart disease, it is important to know whether myocardial ischemia changes the threshold for ventricular defibrillation under experimental conditions simulating automatic internal defibrillation. We determined changes in ventricular defibrillation threshold following coronary occlusion, using an electrode catheter designed for use with an automatic implantable defibrillator. Acute myocardial ischemia was produced without thoracotomy in 10 dogs (experimental group) by embolization with a plastic bead injected via a catheter into the left coronary artery. A control group of 4 dogs had only saline injected into the artery. Defibrillation threshold was measured at 15-min intervals from 1 hour before embolization to 2 hours after embolization. In the control group, voltage, current, energy, and impedance were unchanged after injection of saline into the coronary artery, and india ink perfusion revealed no ischemic areas. In the experimental group postembolization threshold current and energy were significantly higher than preembolization values: 0.47 vs 0.40 A/kg and 1.01 vs 0.80 J/kg, respectively (p less than 0.01). The magnitude of the peak change in threshold current after embolization was positively correlated (r = 0.79) with the size of the ischemic zone, determined by weighing unstained areas after india ink perfusion. Defibrillation threshold for a catheter electrode configuration increases for at least 2 hours following onset of acute myocardial ischemia. This finding must be accounted for in the design and use of an automatic implantable defibrillator.