A number of 3-substituted pyrimido[5,4-b]indole-2,4-diones (7-23) were evaluated for their in vitro alpha 1 adrenoceptor affinity by radioligand receptor binding assays. Some compounds bearing a (phenylpiperazinyl)alkyl side chain were potent alpha 1 adrenoceptor ligands. The most active derivative in displacement of [3H]prazosin from rat cortical membranes was 3-[2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl]pyrimido[5,4-b]indol e- 2,4-dione (10) (Ki = 0.21 nM). Discrete modifications in the structure resulted in higher selectivity (greater than 10,000 times) for alpha 1 than alpha 2, beta 2, and 5HT1A receptors. Some compounds also had affinity for the 5HT1A receptor. The most selective alpha 1 ligand was 3-[2-[4-(2-chlorophenyl)piperazin-1-yl]ethyl]pyrimido[5,4-b)indole - 2,4-dione (13).