Platelet interaction with cerebral microvessels was studied in rats after global brain ischemia. Studies were performed with a model of global central nervous system ischemia produced by cardiocirculatory arrest in normothermic rats. The ischemic period of 5-min was followed by times of recirculation lasting 5 and 15 mins, 1, 3, 6, 24, and 48 hrs, and 6, 10, and 12 months. After different periods of survival, blood vessels from brains were examined by transmission electron microscopy. Our investigation revealed numerous platelet aggregates of varying sizes within both arterial and venous intraparenchymal vessels. At the same time, we also noted platelets in various stages of disintegration. Platelets close to the endothelial cells were often degranulated, with shape changes including pseudopodia. Aggregates of platelets were focal, random, and more widespread occurring in the cerebral cortex, thalamus, basal ganglia, hippocampus, and cerebellum. We also observed recent aggregates of platelets and thrombi, which presented varying degrees of degranulation. In one time period, some platelets were found outside the brain vessels (in the perivascular space after 24 hrs of survival). We present evidence that platelet aggregation was repeated a long time after the ischemic incident, i.e., after one year. The main result was a prominent and local accumulation of platelets in microvessel branches or regions of vessel bifurcations, which correlated well with blood-brain barrier alterations observed previously in this model. The platelet aggregations increased in frequency with longer periods of recirculation. We noted that local platelet adhesion/aggregation was present after brain ischemia requiring no denudation or exposure of the basal lamina.(ABSTRACT TRUNCATED AT 250 WORDS)