Capillaries of severely ischemic myocardium develop marked alterations in structure and permeability [1–7]. The pathogenesis of these changes is largely unknown; moreover, it has been the subject of relatively little investigative effort. The studies described below were not designed to evaluate capillary permeability in ischemia. Rather, they were aimed at learning the molecular events or series of events which cause ischemic injury to become lethal. Nevertheless, during the course of these experiments, striking changes in capillary structure and function were demonstrated in severely ischemic left ventricular myocardium before and after reperfusion of arterial blood. These changes are the subject of this paper.