The specific binding of the alpha 2-adrenoceptor antagonists [3H]yohimbine and [3H]rauwolscine to membranes prepared from rat cerebral cortex and human platelets was rapid, reversible, saturable and of high affinity. Both ligands labelled an identical population of sites in cerebral or platelet membranes though the affinities of both agents were significantly lower in brain. The specific binding of [3H]yohimbine and [3H]rauwolscine in both tissues was displaced by various adrenergic agents in a manner that suggests that the labelled sites probably represent the alpha 2-adrenoceptor. There were, however, significant differences in the affinities of certain alpha-antagonists between cerebral and platelet preparations, even though these agents generated displacement curves with slopes close to unity in both tissues. Thus, whereas yohimbine, rauwolscine, WB4101 and corynanthine were all weaker in cerebral cortex, prazosin was significantly more potent at cortical than in platelet membranes. A possible heterogeneity of alpha 2-adrenoceptors is discussed.