We have compared the influence of a series of benzodioxane alpha-adrenergic antagonists on 3H-WB-4101 and 3H-clonidine binding to alpha-receptor sites in the brain and peripheral tissues with their pharmacological properties. The drug specificity of 3H-WB-4101 binding is quite similar in central and peripheral tissues. Pharmacological potencies of benzodioxanes at postsynaptic alpha-receptors in the rat vas deferens correlate with potencies at 3H-WB-4101 but not at 3H-clonidine binding sites. These findings suggest pharmacological effects of these drugs are mediated by "alpha-1 postsynaptic receptors" labeled by 3H-WB-4101. For several benzodioxanes absolute pharmacological potencies at postsynaptic alpha-receptors of the rat vas deferens are substantially less than their potencies at 3H-WB-4101 sites. The potencies of benzodioxane analogues at 3H-clinidine binding sites are similar to their pharmacological potencies at presynaptic autoreceptors in the rat vas deferens.