Alpha 2-adrenoceptors on canine platelet membranes were characterized by [3H]-yohimbine binding. Binding of [3H]-yohimbine to the membranes was rapidly saturated, stable and reversible with high affinity. Dissociation constant (KD) calculated from Scatchard analysis of the equilibrium experiments was 1.89 +/- 0.20 nM which was in good agreement with KD (1.97 +/- 0.60 nM) obtained in the kinetic experiments. Maximal binding sites of the ligand (Bmax) was 249 +/- 20 fmoles/mg protein. Adrenergic antagonists inhibited the ligand binding in following rank order of potency; yohimbine greater than phentolamine greater than phenoxybenzamine greater than corynanthine greater than prazosin. This order was in good correlation with that for these drugs in the inhibition of platelet aggregation. Hill coefficients for the displacement curves of the ligand binding by adrenergic agonists were less than 1.0 which were converted into near unity in the presence of 5'-guanylyl-imidodiphosphate (100 microM). These results demonstrate that canine platelet membranes have alpha 2-adrenoceptors and suggest that the binding of these receptors to adrenergic agonists is regulated by guanine nucleotides.