Cerebral ischemia leading to infarction was produced in rats by intravascular thrombosis induced by a photochemical reaction between systemically injected rose bengal and green light (560 nm) transmitted through the intact skull for a 2-min period. At 2 or 15 min following photochemical sensitization, animals were perfusion-fixed for scanning (SEM) and transmission (TEM) electron microscopic analyses of the cerebral vasculature. At 2 minutes, ultrastructural examination of cortical regions destined to undergo infarction revealed numerous platelet aggregates within both pial and intraparenchymal vessels. Platelets close to the endothelial walls were routinely degranulated with pseudopodia. Endothelial cells were frequently swollen and contained dilated mitochondria and granular endoplasmic reticulum. The endothelial luminal membrane structure was shown by high-power TEM to be focally damaged. If brain temperature was reduced by 4 degrees C during the photochemical sensitization period, the platelet response was inhibited without interfering with other ultrastructural changes. These results are consistent with the hypothesis that photochemically induced endothelial alterations stimulate platelet activation and implicate abnormal endothelial function as a primary event in the pathogenesis of photochemically induced cerebral infarction.