In helically-cut strips of cerebral arteries isolated from dogs, analogues of 5-hydroxykynurenamine (5-HK), including 2-(3'-aminopropyl)-aniline (Cpd. I), 2'-amino-3-dimethylamino-3'-hydroxypropiophenone(CPD. II), 2'-amino-3-dimethylamino-5'-hydroxypropiophenone (Cpd. III) and 2',3-diamino-propiophenone (kynurenamine), caused a dose-related contraction which was antagonized by treatment with methysergide. The potency for inducing contractions was in the order of 5-hydroxytryptamine greater than 5-HK greater than Cpd. III greater than kynurenamine, Cpd. I and Cpd. II. Treatment with the 5-HK analogues antagonized the contractile response to 5-hydroxytryptamine in a dose-dependent manner, the antagonistic potency being in the order of 5-HK greater than Cpd. III greater than kynurenamine, Cpd. II greater than Cpd. I. Alterations in the hydroxy group on the benzene ring and/or radicals of long side chain of 5-HK attenuated the agonistic and antagonistic actions of 5-HK; however, the attenuation of these actions differed. Thus, the radicals appear to be involved in the agonistic and antagonistic actions to a different extent.