Objective To investigate the development and kinesis of NOS isoforms in different reperfusion time in a model of reversible middle cerebral artery occlusion(MCAO) in rats.Methods Male Sprague-Dawley rats were anesthetized with isoflurane and subjected to an intraluminal MCAO for 90 minutes and reperfusion thereafter.The activity of the different isoforms of NOS(cNOS and iNOS) from the right and left hemispheres was measured at 1,2,6,9,12,and 24 hours reperfusion respectively.Following neurological deficit evaluation,brains were removed and sectioned at different reperfusion time point respectively.The infarct volume was determined with an imaging analysis on the brain sections stained with TTC.Results Conditions of severe MCAO induced ischemia and reperfusion excessively activate NOS,resulting in activities of cNOS and iNOS were significantly increased in the ischemic hemisphere at all time points of reperfusion for 24 h. The peak activity of cNOS was at 6h after reperfusion while that of iNOS appeared at 9 h.The infarct volume was increased parallel to the increasing of NOS.Conclusion cNOS from endothelial cells(eNOS) and neurons(nNOS) is constitutively expressed enzymes,whose activity is stimulated by early ischemic and reperfusion injury and get peak activity in the brain at 6 h after reperfusion iNOS,which is very little present in healthy tissue,is induced shortly after ischemia but its peak activity appeared at 9 hours after reperfusion.Both cNOS and iNOS contribute to secondary late-phase ischemic damage.