L-Arginine (> or = 30 mg kg-1, i.v.), but not D-arginine (300 mg kg-1) administered 5 min after unilateral common carotid/middle cerebral artery occlusion increased regional cerebral blood flow (rCBF) within the dorsolateral ischaemic cortex in spontaneously hypertensive rats. L-Arginine (300 mg kg-1) increased rCBF from 22 +/- 2.7 to 33 +/- 4% of baseline as measured by laser-Doppler flowmetry. This increase may explain the ability of L-arginine to reduce infarct size following focal cerebral ischaemia, as reported previously. The mechanism appears to be mediated by nitric oxide since topical L-NAME (1 microM), a nitric oxide synthase inhibitor, decreased pial arteriole calibre from 115 +/- 2.2 to 106 +/- 0.9% of baseline following L-arginine infusion (300 mg kg-1).