Cerebral vasospasm following subarachnoid hemorrhage was induced in the squirrel monkey in order to evaluate the involvement of cerebrovascular sensory nerves in the development of the vasospasm. A unilateral surgical section of the trigeminal nerve at post- but not at pre-Gasserian level caused constriction of the major ipsilateral cerebral arteries. A pre- or postganglionic trigeminal lesion induced an increased glucose uptake globally without influencing the cerebral blood flow. Following a subarachnoid hemorrhage, the decrease in cerebral blood flow was similar of that seen in control animals, while post-ganglionically lesioned animals had an additional increase in glucose uptake. Intrathecal injection of gamma-globulin against substance P prevented the occurrence of vasospasm and the decrease in cerebral blood flow, while calcitonin gene-related peptide (CGRP) anti-gamma-globulin injection significantly reduced the resting vessel diameter and did not influence spasm development. It is concluded that a nervous reflex mechanism could underlie cerebral vasospasm. The cerebrovascular sensory nerves have both a peripheral and a central function. A peripheral or axon reflex mechanism exerts a tonic effect on the cerebral arteries. Central neurotransmission seems to be involved in the regulation of cerebral metabolism and possibly in the coordination of cerebral blood flow and glucose metabolism. CGRP could be the transmitter involved in a peripheral axon reflex and substance P might be the neurotransmitter conveying information to the brainstem vascular centers.