The selectivity of tizanidine to the imidazoline-receptor and alpha 2-adrenoceptor recognized by 3H-p-aminoclonidine was examined in rat kidney membranes and was compared with those of other imidazoline compounds. Tizanidine bound to the imidazoline-receptors with approximately 20 times higher affinity than the alpha 2-adrenoceptors. The order of relative selectivity to imidazoline receptor was tizanidine greater than oxymetazoline greater than clonidine greater than naphazoline, where clonidine showed an equal affinity to both receptors. Tizanidine may act more potently on the imidazoline-receptors than the alpha 2-adrenoceptors.