Cisternal injections of blood in the rat and squirrel monkey produce an angiographically demonstrable biphasic vasospasm with a maximal late spasm at two days in the rat and six days post-subarachnoid hemorrhage (SAH) in the monkey. The SAH induces a decrease in cerebral blood flow of about 25% and a corresponding increase in glucose uptake of between 30% and 50%. In about half of the animals low-flow areas were noted in the cortex and the basal ganglia with a corresponding marked increase in glucose uptake. Lesioning of the A2-nucleus, its ascending pathway or the median eminence prevents the occurrence of spasm. Similarly, treatment with a substance P antagonist or gammaglobulin against substance P prevents or significantly reduces the degree of spasm. A unilateral post-ganglionic trigeminal lesion causes an ipsilateral constriction of the cerebral arteries of 27%, while a preganglionic lesion does not affect the baseline diameter. A pre- or post-ganglionic trigeminal lesion induces an increase in glucose uptake globally of about 50% without influencing cerebral blood flow. Following SAH the decrease in blood flow in both groups of lesioned animals is similar to that seen in controls. After SAH there is no further change in glucose uptake in the animals with a preganglionic lesion, while in the post-ganglionically lesioned animals there is an additional increase in glucose uptake of about 50% as compared to controls or the animals with a preganglionic lesion.