NADPH-diaphorase histochemical technique was applied to demonstrate the catalytic activity of nitric oxide synthase (NOS) in the nerve fibers supplying some porcine cerebral blood vessels, as well as in ganglia thought to be their possible sites of origin. Five sexually mature Large With Polish race sows were used. The following blood vessels and their branches were studied: the basal cerebral artery, medial cerebral artery, arteries of the circle of Wills as well as arteries located in the arachnoidea. The activity of NADPH-d was visualised in whole-mount preparations from the above listed blood vessels. The presence of NADPH-d was additionally studied in the ganglia of trigeminal nerves, the sensory ganglia of vagus nerves, the pterygopalatine and cranial cervical ganglia. NADPH-d activity was found in nerve fibres supplying all the studied arteries. Larger blood vessels, the basal cerebral artery, medial cerebral artery and arteries of the circle of Wills possessed very dense NADPH-d-positive nerve plexuses while arachnoidal arteries were poorly innervated by only single nerves. The vascular nerve fibers formed bundles varying in thickness, from very thick bundles often interchanging nerve fibers to quite thin fascicles. Thick bundles were absent from the walls of medium sized vessels and small meningeal arteries where only smaller fascicles or single fibres occurred. NADPH-d-positive neurons and nerve fibres were found in all the ganglia investigated. However, pronounced differences in the number of the positive nerve structures were observed between the ganglia. In the pterygopalatine, trigeminal and sensory ganglia of the vagal nerve the vast majority of neurons were NADPH-d-positive. Numerous NADPH-d-positive nerve fibers occurred within the pterygopalatine and trigeminal ganglion while sensory ganglia of the vagal nerve comprised smaller number of fibres. Small numbers of the neurons and moderate numbers of the nerve fibres occurred in the cranial cervical ganglion. The intensity of NADPH-d reaction in the endothelium was constant independent of the size of the vessels studied.