We evaluated the inhibitory activity of a novel prostacyclin analog, OP-2507 (15-cis-(4-n-propylcyclohexyl)-16,17,18,19,20-pentanor-9-deo xy-6,9 alpha- nitriloprostaglandin F1 methyl ester) on the brain edema induced by occlusion of the middle cerebral artery in cats. Middle cerebral artery occlusion for 4h caused a decrease of regional cerebral blood flow. The specific gravity of the cerebral cortex measured 4h after the middle cerebral artery occlusion as an index of cerebral edema showed a significant reduction. Intravenous infusion of OP-2507 at infusion rates of 10 and 50 ng/kg per min was started 30 min before the middle cerebral artery occlusion and was continued for 4.5 h. While OP-2507 did not affect the blood pressure, heart rate and regional cerebral blood flow before and after the middle cerebral artery occlusion, the reduction of the specific gravity of cerebral cortex was significantly prevented by OP-2507 treatment at both doses. Prostacyclin prevented the reduction of the specific gravity only at the higher dose of 50 ng/kg per min. The present results indicate the potential usefulness of OP-2507 in acute ischemic cerebral disorders.