The physiological effects of the sulfoconjugates of epinephrine, norepinephrine, and the 3-O-methylated catecholamines, metanephrine, normetanephrine, and methoxytyramine were examined with regard to their α2-adrenoceptor binding properties and aggregation activity in human platelets. Sulfoconjugation of catecholamines resulted in the loss of both their competitive potency for [3H]yohimbine binding and their influence on platelet aggregation. O-Methyl substituted catecholamines showed attenuation of their α2-adrenoceptor binding affinities when compared with those of the corresponding non-etherified amines. Unlike the free amine epinephrine, which stimulated platelet aggregation, the O-methylated catecholamine derivatives inhibited aggregation. Inhibition was dose-dependent and restricted to the α2-adrenoceptor mediated aggregation response stimulated by epinephrine (1 μM) or potentiated by subthreshold concentrations of epinephrine (30–300 nM) in the presence of subaggregatory doses of vasopressin (10–30 nM). Collagen- and ADP-induced platelet aggregation was not affected. The hydrophilic β-antagonist CGP 12177 displayed no effects. However, high concentrations (0.1 mM) of both isomers of the strongly lipophilic β-adrenoceptor antagonist propranolol inhibited the actions of all aggregators by stabilizing the membrane. Such a nonspecific membrane interaction of the methylated catecholamines could be excluded because of their low lipid solubility calculated in a n-octanol–phosphate buffer system at pH 7.4. We suggest therefore that methylated catecholamines are biological α2-adrenoceptor antagonists acting on α2-adrenoceptor stimulated reactions of human platelets. Whether this receptor antagonism is relevant to other human tissues needs clarification. Sulfated catecholamines, however, are wholly ineffective at this receptor site and may constitute a pathway to control the concentration of the active free catecholamines.Key words: sulfoconjugated catecholamines, methylated catecholamines, α2-adrenoceptors, human platelet aggregation, dopamine receptors.