Objective:We tested whether AR-R 17477, a selective inhibitor of neuronal nitric oxide synthase, reduces brain injury in rats subjected to permanent focal ischemia.Design:Randomized within cohort; nonblinded study.Setting:University basic science laboratory.Subjects:Halothane-anesthetized male Wistar rats (n = 53).Interventions:Rats were treated with either intravenous saline (diluent) or AR-R 17477 (1 or 3 mg/kg) 30 mins before or 60 mins after the onset of permanent focal cerebral ischemia. Infarction volume was determined at 18 or 48 hrs of ischemia.Measurements and Main Results:Pretreatment with 1 mg/kg AR-R 17477 was associated with a decreased infarct volume (2,3,5-triphenyltetrazolium chloride staining) in the striatum (saline, 81 ± 7 mm3; AR-R 17477, 55 ± 3 mm3) but not in the cortex at 18 hrs of occlusion (saline, 302 ± 29 mm3; AR-R 17477, 237 ± 36 mm3). However, this therapeutic effect of AR-R 17477 was no longer evident if the rats were allowed to survive for 48 hrs before analysis of infarction volume. In fact, in this separate cohort of animals, three of eight AR-R 17477-treated and five of eight saline-treated rats died before completing 48 hrs of ischemia. Efficacy of AR-R 17477 was completely absent (even at 18 hrs of ischemia) when drug treatment was delayed until 1 hr after the onset of ischemia. Infarction volume at 18 hrs of ischemia was similar between rats treated with saline, 1 mg/kg (cortex, 229 ± 43 mm3; striatum, 67 ± 8 mm3) or 3 mg/kg AR-R 17477 (cortex, 284 ± 34 mm3; striatum, 75 ± 5 mm3). In addition, only one of eight rats treated with 3 mg/kg AR-R 17477 at 1 hr of ischemia survived 48 hrs of occlusion, compared with three of eight rats treated with saline.Conclusions:Neuronally generated nitric oxide is a mediator of brain injury during permanent focal ischemia in rats. However, severity of the ischemic insult appears to limit the therapeutic efficacy of the specific neuronal nitric oxide synthase inhibitor, AR-R 17477.