There are no useful markers in blood of nitric oxide (NO)-mediated brain damage. Because l-arginine (l-arg) is the only known substrate for NO generation, the authors investigated the plasma profile of l-arg after cerebral ischemia, and the relationship of L-arg concentrations in blood with stroke outcome and infarct volume in a clinical and experimental study. l-Arg levels were determined with high-performance liquid chromatography in blood and CSF samples obtained on admission, and in blood 48 hours after inclusion, in 268 patients admitted with a hemispheric ischemic stroke lasting 8.2 +/- 5.9 hours. Infarct volume was measured by days 4 to 7 using computed tomography. Plasma l-arg profiles were analyzed in a separate group of 29 patients seen within 8 hours of onset (median, 4.5 hours) and in 24 male Fischer rats treated with subcutaneous vehicle or 20-mg/kg 1400W (a specific inducible NO synthase inhibitor) every 8 hours for 3 days after performing sham or permanent middle cerebral artery occlusion. Plasma l-arg concentrations decreased after the ischemic event, both in patients and rats, and peaked between 6 and 24 hours. In patients, there was a highly correlation between l-arg levels in CSF and plasma at 48 hours (r = 0.85, P<0.001). CSF and plasma l-arg concentrations negatively correlated with infarct volume (r = -0.40 and r = -0.35, respectively, P<0.001), and were significantly lower in patients with early neurologic deterioration and in those with poor outcome (Barthel index <85) at 90 days (P<0.001). In rats, the administration of 1400W resulted in a 55% significant reduction of infarct volume measured 72 hours after permanent middle cerebral artery occlusion, an effect that correlated with the inhibition caused by 1400W on the ischemia-induced decrease of plasma l-arg concentrations at 6 to 24 hours after the onset of the ischemia. Taken together, these data indicate that determination of l-arg levels in blood might be useful to evaluate the neurotoxic effects of NO generation. These findings might be helpful to guide future neuroprotective strategies in patients with ischemic stroke.