In order to study the characterization and localization of [ 3 H]RX-821002 (2-methoxy-idazoxan) binding to α 2 -adrenoceptor subtypes in several regions of the human brain, we have carried out competition studies using both autoradiography and membrane binding assays. The α 2 A -adrenoceptor subtype was found to be predominant in the different layers of the frontal cortex, cerebellum and hippocampal formation, while in the neostriatum it was the non-α 2 A - (α 2 B - and α 2 C -) adrenoceptor subtype. In the frontal cortex, in addition to binding to the α 2 A -adrenoceptor subtype, [ 3 H]RX-821002 bound also to a small portion of α 2 B -adrenoceptors in layer III, and to an unidentified binding site in the external layers. In the hippocampus, both α 2 A - and non-α 2 A - (α 2 B - and α 2 C -) adrenoceptors were labelled in the dentate gyrus and the CA 1 field, together with 5-HT 1 A receptors. 5-HT 1 A receptors were labelled predominantly in the stratum pyramidale layer. These results, in addition to delineate the relative presence of α 2 -adrenoceptor subtypes, indicate that caution is needed when analyzing RX 821002 binding to human brain tissue.