Epinephrine induces human platelet aggregation by interacting with alpha-adrenergic receptors. These sites were demonstrated by radioligand-binding techniques using the new antagonist ligand, [3H]yohimbine. The sites labeled by [3H]yohimbine had the specificity of alpha 2-receptors with the affinity of yohimbine much greater than prazosin. Epinephrine-mediated inhibition of prostaglandin E1-stimulated adenylate cyclase activity in human platelet lysates was also found to have an alpha 2-receptor specificity. Competition curves of antagonists with [3H]yohimbine indicated a homogeneous population of alpha 2-receptors. In contrast, competition curves of a series of full and partial agonists with [3H]yohimbine were resolved into two distinct affinity states; the ratio of the dissociation constants of agonists for the low and high affinity states was positively correlated with the agonist's intrinsic activity for inhibition of adenylate cyclase. Guanine nucleotides were found to destabilize the high affinity form of the alpha 2-receptors. At high nucleotide concentrations, all high affinity states of the receptor were converted to the low affinity form. The formation of the high affinity agonist-binding state may reflect an interaction between the agonist-receptor complex and an additional membrane component, and probably reflects events involved in alpha 2-receptor-adenylate cyclase coupling.