Hu m an (h) aC G R P an d PCGRP have p re v io u s ly b e e n sh o w n to in d u c e vasodila tion by activation o f haCGRPg-37 sensitive CGRP receptors on segm ents o f guinea pig basilar a rte ry in v itro .1 In this study we have investigated levels o f cAMP and cGM P in segm ents o f guinea pig basilar artery following adm inistration o f haCGRP an d hpCG RP in p resence o r absence o f haC G R Pg.37. We additionally investigated w hether the vasodilatory action o f h a - and hpCG R P are m ediated via the endothelium , n itr ic ox ide syn thase (N O S ), guanylate cyclase stim ulation or by a cyclooxygenase involved mechanism. This was investigated by determ ination o f CGRP induced vasom otor responses following removal o f endothelium or treatm ent by N G-nitro-L-arginine methyl ester (L-NAME), methylene blue or indom ethacin.