New analogs of desoxycatecholimidazoline were synthesized for elucidating the steric requirements for the activation of alpha-1 adrenoreceptors. The compounds tested on rat thoracic aorta in this study were: desoxycatecholimidazole with and without bridge carbon hydroxyl group, analogs of desoxycatecholamidazoline with fluorine substitution at position 2, 5 or 6 of the catechol ring and hydroxybenzyl group at carbon-4 of the imidazoline part of the molecule. The addition of a double bond in the imidazoline to give an imidazole results in a decrease in potency and the introduction of benzylic hydroxyl group also reduces its activity by 4- and 6-fold, respectively. 2-Fluoro and 5-fluoro catecholimidazoline possess full agonist activity; their potencies being even higher than the parent molecule. The 6-fluoro analog is a partial agonist inasmuch as it produces a response that is only 30% of the maximum response produced by other analogs of imidazoline. In the present study, 4-substituted imidazolines retain their agonist activity, although weaker than desoxycatecholimidazoline. The potency of R- and S-isomers of 4-substituted catecholbenzyl imidazoline were similar. Although these isomers exhibit apparent chemical similarity to catecholamines, small differences between the activity of stereoisomers indicate that the mode of interaction of these molecules at alpha adrenoreceptor may differ from that of stereoisomers of epinephrine.