Platelets from various mammalian species differ in the response to adrenaline mediated by activation of alpha 2-adrenoceptors. In contrast to platelets from rabbit, dog and man, adrenaline did not potentiate the ADP-induced aggregation of rat and guinea pig platelets. Binding assays with 3H-yohimbine, a selective alpha 2-adrenoceptor antagonist, showed that the radioligand was not bound specifically to rat and guinea pig platelets. The specific high affinity binding of 3H-yohimbine to intact human, dog and rabbit platelets was saturable and reversible. The Scatchard analyses of 3H-yohimbine binding showed that the number of binding sites and the affinity for the radioligand decreased in the order man greater than dog and dog greater than rabbit. The reduction in alpha-adrenoceptors on platelets seems to account for the diminished responsiveness to adrenaline.