Sequential ultrastructural lesions of rabbit myocardial microvasculature after a single dose (1008 or 1300 rads) of local x-irradiation are described. Vascular permeability status was assessed through use of ferritin and colloidal carbon. Endothelial cell swelling and increased vascular permeability were the most conspicuous lesions during the first week following irradiation. Increased vascular permeability, as indicated by the observed ferritin and carbon distribution, appears to be a result of altered pinocytotic transport and widening of endothelial junctional gaps. These lesions, on day 14 and later, were followed by basement membrane thickening, endothelial cell extrusions and bleb formation, platelet sequestration, abnormal endothelial cell phagocytosis and appearance of myelin-like figures within the endothelial cells.