INTRODUCTION--Cerebral blood vessels are innervated by sympathetic nerve fibres storing neuropeptide Y (NPY), parasympathetic nerves storing acetylcholine, vasoactive intestinal peptide (VIP) and sensory afferent fibres containing calcitonin gene-related peptide (CGRP), substance P (SP) and neurokinin A. In experimental studies on subarachnoid haemorrhage (SAH) there are indications that perivascular peptides are involved. In the present study we have in man measured the levels of NPY, VIP, SP and CGRP in brain vessels of patients that have suffered a fatal SAH and compared this with the levels encountered in subjects that died of an extracerebral cause. MATERIAL AND METHODS--Vessels from patients who have died from SAH or nonSAH were obtained during autopsy performed within 24 hrs after death. The peptides were extracted and fractionated with reversed phase liquid chromatography (HPLC). The levels of NPY, VIP, SP, and CGRP were measured with radioimmunoassay. Vasomotor responses of human cerebral arteries were performed using a sensitive in vitro system. RESULTS--Human cerebral vessels contained NPY, VIP, CGRP and SP which eluted at the same positions as the authentic peptides. The level of CGRP was significantly lower (p < 0.01) in arteries removed from SAH patients as compared to control subjects. The level of SP was not changed, if anything it tended to be increased after SAH. The levels of NPY and VIP were not significantly altered after SAH. In isolated brain vessels alpha-CGRP was a potent vasodilator of arteries precontracted with whole blood, prostaglandin F2 alpha or endothelin. It had a poor effect on vessels precontracted with 60 mM potassium. CONCLUSION--The evidence suggest that the trigemino-cerebrovascular system, storing CGRP and SP, is to a differential degree involved in the pathophysiology of SAH in man and supports the hypothesis of an exhaustion of CGRP as one important factor in the development of late spasm occurring after SAH.