Prostaglandin F2? ( PGF2? ) in the brain cortex was measured in mice which were devided into 3 groups, ie sham operation group, cerebral ischemia group and reperfusion after cerebral ischemia group, treated with saline or clonidine ( 0 .15~1 .Omg/kg ) , respectively. The results showed that the PGF2? level in the ischemia group treated with saline was higher than sham operation group, but the PGF2? level in reperfusion group was the highest in all groups. Compared with saline, clonidine decreased the PGF2? levels either in the ischemia group or the reperfusion group significantly . This may be useful to attenuate the brain lesion caused by PGF2? .