A new class of alpha2-adrenergic receptor antagonists, the 2-(aminobenzyl)-1,2,3,4-tetrahydroisoquinolines 4 and their derivatives, is described. Two synthetic routes are reported. An investigation of the structure-activity relationships including various substitutions of the isoquinoline moiety and the benzyl group is discussed. The affinity and selectivity for both alpha1- and alpha2-adrenoceptors was defined by studying the displacement of [H-3]-prazosin (alpha1-sites) and [H-3]-yohimbine (alpha2-sites) from rat brain membranes. The 2-(2-amino-3,4-dimethoxybenzyl)-6-methoxy-1,2,3,4-tetrahydroisoquinoline 4a presented affinity and selectivity close to yohimbine. In functional experiments the alpha-adrenoceptor blocking properties of 4a have been evaluated on isolated rat aorta and on the twitch responses of the isolated rat vas deferens.