In order to study the role of opioid receptors in two models of experimental hypertension the binding of 3H-naloxone to membranes prepared from discrete brain regions and spinal cord was determined. Renal hypertensive rats (RHR) were found to have a greater density of 3H-naloxone binding sites in the hippocampus and hypothalamus when compared to spontaneously hypertensive rats (SHR) and normotensive Wistar rats (NR). The apparent dissociation constant (Kd) for 3H-naloxone binding did not differ between groups.