The intrathecal administration of clonidine to rats results in profound antinociception which is thought to be mediated through an interaction of the agonist with spinal alpha-2 adrenergic receptors. However, clonidine has been shown to also interact with nonadrenergic imidazoline receptors. Consequently, this study was undertaken to determine if nonadrenergic imidazoline receptors are present in the rat spinal cord, and the extent to which they are involved in the antinociceptive action of spinally administered clonidine. By using the tail-flick test, the antinociceptive action of spinally administered clonidine was found to be blocked completely by the intrathecal administration of the imidazoline idazoxan. Similarly, yohimbine (a nonimidazoline alpha-2 adrenergic antagonist) also blocked completely the antinociceptive action of clonidine. Results of radioligand binding studies demonstrated that norepinephrine did not interact with approximately 20% of all specific spinal sites labeled by 4 nM [3H]clonidine, indicating the presence of nonadrenergic spinal sites. Affinity data obtained from competition binding assays demonstrated that the spinal nonadrenergic sites labeled by [3H]clonidine possess little affinity for yohimbine. Therefore, nonadrenergic imidazoline receptors are not involved in the antinociceptive action of spinally administered clonidine.