The imidazoline (I receptors) receptor family has received extensive attention since its identification by Bousquet et al., Coupry et al., and Ernsberger et al. in the 1980s. Agmatine has been proposed to be the endogenous ligand for the I receptors, but there is question whether it is the sole endogenous agent. Previous ligands interact with other receptors including α2 adrenoceptors, precluding confirmation of the specific receptor responsible for observed physiologic responses. Our interest arose from searching for selective α2 agonists with no imidazoline receptor activity, hypothesizing that a conformationally restrained framework could enhance selectivity to reduce elevated intraocular pressure without side effects like blood pressure reduction. During studies, we discovered AGN 192403 (3), the first agent equipotent to moxonidine in imidazoline receptor binding assays and nonpotent in adrenergic binding assays. Surprisingly, this potent, I1-specific agent was devoid of all physiologic responses: it did not affect blood pressure, intraocular pressure, or induce sedation in animal models, nor did it antagonize or potentiate effects of other imidazoline receptor agonists. This may suggest a new binding site or that the imidazoline site defined by binding assays is a nonfunctional binding site. Compound 3 was synthesized via condensation of 2-methylpropionaldehyde with nitromethane, treatment with methanesulfonyl chloride, Diels-Alder reaction with cyclopentadiene, and Pd-C-catalyzed hydrogenation. Binding assays showed high affinity for I1 (Ki 42 nM) and no activity on α adrenergic receptors. In vivo, it had no effect on blood pressure even at 5000 µg/kg in cynomologus monkeys (while clonidine and moxonidine reduced blood pressure at lower doses), no effect on intraocular pressure, and was nonsedating. Our data suggest the I1 site is a unique binding site but not a functional receptor, supporting recent studies that rilmenidine's effects may be mediated by α2 adrenoceptors.