The effect of intravenous administration of NG-monomethyl-L-arginine (L-NMMA, 30 mg kg-1), a specific inhibitor of nitric oxide (NO) synthesis, on the autoregulation of local cerebral blood flow (LCBF) was examined in the rat using the [14C]iodoantipyrine autoradiographic method. LCBF was significantly lower in various superficial regions such as the cerebral cortices and cerebellar cortex and in several deep brain regions in animals with haemorrhagic hypotension induced after L-NMMA infusion (the L-NMMA + HEM group) compared with animals without haemorrhagic hypotension after L-NMMA infusion (the L-NMMA group). The present findings suggest that NO synthesis may play a crucial role in the autoregulation of LCBF in response to a reduction in blood pressure in the cerebral cortices, cerebellar cortex and several deep brain regions.