The α-adrenoreceptor-related dissociation constant and intrinsic efficacy of the stereoisomers of epinephrine were determined on the rabbit aortic strip. (–)-Epinephrine showed higher affinity than (+)-epinephrine or the desoxy analogue epinine. Efficacy of (+)-epinephrine relative to (–)-epinephrine was low (relative efficacy, er = 0.44). Epinine had about the same affinity and efficacy as (+)-epinephrine. When the α-adrenoreceptors were protected against dibenamine by an equimolar concentration of the isomers, (–)-epinephrine was found to be more effective than (+)-epinephrine or epinine. These results suggest that the stereoisomers of catecholamines differ not only in affinity but also in intrinsic e