The purpose of this study was to test the hypothesis that the efficacy of 7-Nitro-indazole, a selective neuronal NOS inhibitor is pH dependent in vivo during focal cerebral ischemia. Wistar rats underwent 2 hours of focal cerebral ischemia under 1% halothane anesthesia. 7-Nitro-indazole, 10 and 100 mg/kg in 0.1 ml/kg DMSO was administered 30 minutes prior to occlusion. Ischemic brain acidosis was manipulated by altering serum glucose concentrations. Confirmation of the effects of these serum glucose manipulations on brain pH_i were confirmed in a group of acute experiments utilizing umbelliferone fluorescence. The animals were sacrificed at 72 hours for histology. 7-NI significantly (P<0.05) reduced infarction volume in both the normoglycemic by 93.3% and hyperglycemic animals by 27.5%. In the moderate hypoglycemic animals, the reduction in infarction volume did not reach significance because moderate hypoglycemia in itself dramatically reduced infarction volume. We hypothesize that a mechanism to explain the published discrepancies on the effects of nNOS inhibitors in vivo may be due to the effects by differences in ischemic brain acidosis on the production of NO.