Nitric oxide (NO) plays an important role in the pathogenesis of neuronal injury during cerebral ischemia. The endothelial and neuronal isoforms of nitric oxide synthase (eNOS, nNOS) generate NO, but NO generation from these two isoforms can have opposing roles in the process of ischemic injury. While increased NO production from nNOS in neurons can cause neuronal injury, endothelial NO production from eNOS can decrease ischemic injury by inducing vasodilation. However, the relative magnitude and time course of NO generation from each isoform during cerebral ischemia has not been previously determined. Therefore, electron paramagnetic resonance spectroscopy was applied to directly detect NO in the brain of mice in the basal state and following global cerebral ischemia induced by cardiac arrest. The relative amount of NO derived from eNOS and nNOS was accessed using transgenic eNOS(-/-) or nNOS(-/-) mice and matched wild-type control mice. NO was trapped using Fe(II)-diethyldithiocarbamate. In wild-type mice, only small NO signals were seen prior to ischemia, but after 10 to 20 min of ischemia the signals increased more than 4-fold. This NO generation was inhibited more than 70% by NOS inhibition. In either nNOS(-/-) or eNOS(-/-) mice before ischemia, NO generation was decreased about 50% compared to that in wild-type mice. Following the onset of ischemia a rapid increase in NO occurred in nNOS(-/-) mice peaking after only 10 min. The production of NO in the eNOS(-/-) mice paralleled that in the wild type with a progressive increase over 20 min, suggesting progressive accumulation of NO from nNOS following the onset of ischemia. NOS activity measurements demonstrated that eNOS(-/-) and nNOS(-/-) brains had 90% and < 10%, respectively, of the activity measured in wild type. Thus, while eNOS contributes only a fraction of total brain NOS activity, during the early minutes of cerebral ischemia prominent NO generation from this isoform occurs, confirming its importance in modulating the process of ischemic injury.