We have investigated the characteristics of [3H]p-aminoclonidine binding to homogenates of cat frontal cortex and of cat spinal cord. At 37 degrees C, saturable [3H]p-aminoclonidine binding displayed rapid association and dissociation kinetics. Scatchard plots of data obtained from either tissue were consistently biphasic. Computer-assisted nonlinear least squares regression analysis based upon a one ligand/two binding site model was employed to resolve these plots into two linear components. The results of this analysis indicate that, for homogenates of either tissue, the KD of [3H]p-aminoclonidine binding, to the high and low affinity binding sites respectively, is approximately 0.5 nM and 9 nM. Mean Bmax values (fmol/mg protein), for the high and low affinity binding sites respectively, of 28 and 149 for homogenates of cat frontal cortex and 7.1 and 56 for homogenates of cat spinal cord were obtained. The results of competitive inhibition experiments, where 0.13 nM [3H]p-aminoclonidine was used, indicate that catecholamines compete for [3H]p-aminoclonidine binding sites in a stereoselective fashion, and that these sites possess a pharmacology characteristic of alpha 2-adrenoceptors. The results of Hill analysis of the competition data suggest that alpha 2-adrenoceptor agonists and alpha 2-adrenoceptor antagonists possess selective affinity for these two alpha 2-adrenoceptor binding sites. The two populations of saturable [3H]p-aminoclonidine binding sites appear to correspond to those alpha 2-adrenoceptor ligand binding sites that have been referred to as the 'super high' and 'high' affinity state of the alpha 2-adrenoceptors.