In cerebral arteries isolated from most of mammals, nerve stimulation produces relaxations in contrast to contractions in peripheral arteries. The relaxant mechanism is found to be non-adrenergic and non-cholinergic, but the neurotransmitter is not clarified until recently. Based on several functional and histological studies with isolated cerebral arteries, nitric oxide (NO) is now considered to be a neurotransmitter of the vasodilator nerve and the nerve has been called a nitroxidergic (nitrergic) nerve. Upon neural excitation, calcium influxed through N-type Ca2+ channels activates neuronal NO synthase, and then NO is produced by the enzyme from L-arginine. The released NO activates soluble guanylate cyclase in smooth muscle cells, resulting in relaxation with a cyclic GMP-dependent mechanism. The functional role and neuronal pathway have also been investigated in anesthetized dogs and Japanese monkeys. The nitroxidergic (nitrergic) nerves innervating the circulus arteriosus, including the anterior and middle cerebral and posterior communicating arteries, are found to be postganglionic nerves originated from the ipsilateral pterygopalatine ganglion and tonically dilate cerebral arteries in the resting condition. Our findings suggest that the nitroxidergic (nitrergic) nerve plays a physiologically important role to maintain a steady blood supply to the brain.