The specific binding of the agonist [corrected], [3H]clonidine, to neural membranes and clonidine-induced mydriasis were used to evaluate the density and sensitivity [corrected] of alpha 2-adrenoceptors in the brain of spontaneously hypertensive rats (SHR) and sex- and age-matched normotensive Wistar-Kyoto (WKY) rats. In hypertensive rats (SHR) the density of alpha 2-adrenoceptors was reduced in the cerebral cortex, hypothalamus and medulla oblongata (20-27%), as was the dose-pupillary response curve for clonidine (1.8-fold). The results demonstrated that this model of genetic hypertension is associated with desensitization of alpha 2-adrenoceptors in the brain.