Time-dependent changes in c-fos-like immunoreactivity (c-fos-LI) were studied in the rat during focal cerebral ischemia and reperfusion after middle cerebral artery (MCA) occlusion. In the permanent ischemia model, the levels of c-fos-LI increased for the first 30 min of ischemia in neuronal nuclei in the lesioned hemisphere. They reached a maximum at 60 min. The level in the parietal cortex (PC) diminished considerably after 120 min, and in the cingulate cortex (CC) it gradually decreased to near the control value at 180 min. Regional cerebral blood flow (rCBF) in the PC fell to 32% and that in the CC fell to 64% of pre-ischemic values after MCA occlusion. Reperfusion induced strong expression of c-fos-LI in the PC and CC after 6 h of reperfusion that followed 30 min of ischemia. The c-fos-LI was effectively reduced by preadministration of the N-methyl-D-aspartate (NMDA) receptor antagonist, ketamine (100 mg/kg, IP). These findings suggest that the expression of c-fos after ischemia may be immediately activated through NMDA receptors and may spread to surrounding regions in a manner sensitive to reductions in rCBF. Reperfusion after ischemia also appears to cause activation of expression of c-fos and of intracellular signal transduction.