Subsequent to previous observations suggesting a close association between the relaxing actions of PGI2 and its increasing effects on cyclic AMP levels in bovine coronary smooth muscle, evidence indicative of a transmitter function of cAMP in PGI2-induced relaxation is presented, by showing that a) in time course experiments in circular strips of bovine coronary arteries the increase in cAMP-levels caused by PGI2 preceded the mechanical response, b) the relaxant as well as cAMP-increasing effects produced by PGI2 were significantly potentiated by the PDE-inhibitor MIX, and c) in a particulate preparation of bovine coronary arteries PGI2 concentration dependently stimulated adenylate cyclase in the presence of GTP.