Several 2-substituted 5, 6, 11, 11a-tetrahydro-1H-imidazo[1', 5' : 1, 6]pyrido (1) and 5, 6, 11, 11b-tetrahydro-1H-imidazo[1', 5' : 1, 2]pyrido[3, 4-b]indole-1, 3(2H)-diones (2) were synthesized and studied by 2D-NMR spectroscopy and difference nuclear Overhauser effect experiments. All the compounds were evaluated for in vitro α₁ adrenoceptor affinity by radioligand receptor binding assays. The most active derivative in displacement of [³H]prazosin from rat cortical membranes was 1b (K_i=219 nM). At 1 μM concentration, compounds 1 and 2 had no effect on the benzodiazepine or 5-HT_1A receptor. The biological activity profile of 1b makes it a possible lead compound for the design of new selective α₁ adrenoceptor ligands.