BACKGROUND AND PURPOSE Polymorphonuclear leukocytes have been implicated in the development of the "no-reflow" phenomenon after focal cerebral ischemia and reperfusion. To further understand the role of granulocytes in microvascular occlusions, the responses of the granulocyte-endothelial cell adhesion molecules P-selectin and intercellular adhesion molecule-1 during middle cerebral artery ischemia and reperfusion were examined in a primate model. METHODS Twelve adolescent male baboons were used for 2-hour middle cerebral artery occlusion (n = 3) or for 3-hour occlusion with 1-hour (n = 3), 4-hour (n = 3), and 24-hour (n = 3) reperfusion, and three separate unoperated primates served as controls. A quantitative immunohistochemical study of the microvascular distribution of P-selectin and intercellular adhesion molecule-1 was performed using 10-microns frozen sections from basal ganglia analyzed with computerized light microscopy video imaging. RESULTS Significant (P < .05) persistent upregulation of P-selectin (beginning during ischemia) and transient upregulation of intercellular adhesion molecule-1 (at 1 and 4 hours of reperfusion) were observed on endothelium of selected post-capillary microvessels of the ischemic lenticulostriate artery territory. Platelet accumulation also occurred in this territory and was responsible for a significant proportion of the nonendothelial P-selectin signal at 24 hours after reperfusion. CONCLUSIONS Focal cerebral ischemia/reperfusion stimulates endothelial P-selectin and intercellular adhesion molecule-1 expression in brain microvessels in the ischemic zone, which may contribute to enhanced leukocyte adherence and persistent activation.