We studied whether delayed posttreatment with the nitric oxide donor 3-morpholinosydnonimine (SIN-1) is effective in reducing the size of the infarct produced by occlusion of the middle cerebral artery (MCA) in spontaneously hypertensive rats (SHRs). SHRs were anesthetized with halothane and intubated transorally. The left MCA was occluded at the level of the inferior cerebral vein. Cerebral blood flow (CBF) was monitored in the ischemic hemisphere by a laser-Doppler flowmeter, and an electroencephalogram (EEG) was recorded. SIN-1 was infused into the left internal carotid artery for 60 min starting 3, 15, 30, 60 or 120 min after MCA occlusion. The hypotension associated with SIN-1 administration was controlled by i.v. administration of phenylephrine. At the end of the infusion, rats were extubated and allowed to recover. Infarct size was measured 24 h later on thionin-stained coronal brain sections by computer-assisted planimetry. SIN-1 infusion 3 min after MCA occlusion enhanced the recovery of CBF and EEG amplitude and reduced the size of the infarct by 30 ± 11% (p < 0.05, analysis of variance). The reduction in infarct size by SIN-1 was still seen when SIN-1 was administered 15, 30, and 60 min after MCA occlusion (p < 0.05). However, administration of SIN-1 2 h after MCA occlusion did not affect the size of the infarct (p > 0.05). We conclude that posttreatment with SIN-1 is effective in reducing focal ischemic damage if this agent is administered up to 60 min after MCA occlusion. Thus, NO donors, alone or in combination with other treatment modalities, may have therapeutic potential in the management of acute ischemic stroke.