These experiments were designed to determine if the opiate antagonist naloxone affects vascular sensitivity in 2K-1C hypertensive rats. Group 1 was 2K-1C hypertensive rats. Group 2 was 2K-1C rats given a naloxone infusion (100 micrograms/h) for 14 days. Group 3 received naloxone without clipping. Group 4 was untreated rats. At day 14 following clipping, systolic blood pressure was increased significantly in the 2K-1C rats. Those infused with naloxone showed a significant attenuation of the increase in blood pressure. Vascular responses to norepinephrine and KCl in the aortae from all groups were tested. Strips from untreated, 2K-1C rats were more sensitive to the contractile effects of norepinephrine than those from naloxone-treated, 2K-1C rats, and from both groups of normotensive rats. Contractile responsiveness to depolarizing concentrations of KCl were not different among the four groups. These data demonstrate that naloxone attenuates the development of renal hypertension and prevents the increase in vascular responsiveness to norepinephrine.