5-HT has been implicated in the pathogenesis of cerebral arterial vasospasm following subarachnoid haemorrhage and in the aetiology of migraine. Within the cerebrovasculature, 5-HT possesses a complex action dependent on the type of 5-HT receptors present. In this review, Andrew Parsons analyses the evidence demonstrating contractile and relaxant 5-HT receptors within different areas of the cerebrovasculature, and examines differences between human and animal cerebral arteries. The extensive regional and species differences are important considerations for the development of future therapeutic agents in cerebrovascular disease.