Change of immunoreactive P-selectin was examined in rat brain after transient middle cerebral artery (MCA) occlusion (O) with anti-P-selectin monoclonal antibody using brain samples of sham control and after ischemia. Temporal, spatial, and cellular changes of immunohistochemical expressions of P-selectin were evaluated with rat brain sections at 2 and 8 h, 1, 3, and 7 days of reperfusion after 1 h of MCAO. Western blot showed a single band at molecular weight of 140 kDa for P-selectin after ischemia. P-selectin immunoreactivity was not normally present in rat brain sections. However, it was expressed mainly in the post-capillary venules of the cerebral cortex and caudate in the MCA territory with a peak at 8 h-1 day. The expression was diminished by 3 days of reperfusion. The present results indicate that P-selectin was expressed from an earlier stage of reperfusion in post-capillary venules, and the expression became maximum at the same time both in the cerebral cortex and caudate.