The alpha 2-adrenergic agonist [3H]clonidine and antagonist [3H]idazoxan also label I1 and I2 imidazoline receptors in striatum membranes. They are investigated here in striata from the dog, rat, mouse, rabbit, calf, monkey, and human. I1 receptors were barely detected in the dog, rat, and mouse and only further examined by competition binding experiments in calf, rabbit, and human. I2 receptors were further examined in all species. The centrally acting vasodilators clonidine and rilmenidine were more potent than moxonidine at the I1 receptors. They displayed low potency for the I2 receptors in all species except the rat. In all species examined, the nonsubstituted imidazoline derivatives idazoxan and RX801077 displayed high affinity for the I1 and I2 receptors. Conversely, both stereoisomers of the alkoxy-substituted imidazoline-derivative efaroxan displayed low affinity. The matching binding characteristics of these compounds further stress the structural similarity of the ligand binding sites of I1 and I2 receptors.