We evaluated the neuroprotective effect of the nitric oxide synthesis inhibitor,NG-nitro-L-arginine in a neonatal hypoxic-ischemic rat model. Unilateral hypoxic-ischemic injury was produced in the brain of 7-d-old rats using a combination of a common carotid artery ligation and a hypoxic (8% oxygen) exposure for 2.5 h. In our experimental condition, rectal temperatures did not differ betweenNG-nitro-L-arginine-treated and saline-injected pups. We killed the animals 72 h later and assessed the hypoxic-ischemic brain damage histologically.NG-nitro-L-arginine (2 mg/kg) administered intraperitoneally 1.5 h before hypoxia resulted in 77% reduction of the infarcted hemispheric volume and 87% reduction of the infarcted striatal volume compared to saline injected controls.NG-nitro-L-arginine given 1.5 h before the insult also significantly prevented hypoxic-ischemic damage in the five hip-pocampal structures examined, dentate gyrus, CA4, CA3, CA1, and subiculum.NG-nitro-L-arginine administered immediately after hypoxia did not prevent hypoxic-ischemic brain damage. These results indicate that nitric oxide plays a key role in producing neonatal hypoxic-ischemic brain damage.