Ring segments of superior mesenteric arteries studied in vitro were used to determine the role of the vascular endothelium in regulating vascular contractile and relaxant sensitivity in deoxycorticosterone acetate (DOCA)-salt hypertension. Wistar rats were given DOCA (20 mg/kg s.c. twice per week) and 1% NaCl drinking water for 5 weeks. In ring segments containing endothelium, there was a decrease in contractile sensitivity to arginine vasopressin, no change in contractile sensitivity to norepinephrine and KCl, and no change in relaxant sensitivity to acetylcholine or isoproterenol in arteries from hypertensive rats compared with normotensive controls. Removal of the vascular endothelium by rubbing had no effect on the contractile response to arginine vasopressin and KCl or the relaxant response to isoproterenol in control arteries. In arteries without endothelium, DOCA-salt hypertension caused a threefold increase in contractile sensitivity for arginine vasopressin, norepinephrine, and KCl; a 50% reduction in maximal relaxation to isoproterenol; and a threefold decrease in relaxant sensitivity to sodium nitroprusside. Indomethacin (10 microM) had no effect on contraction or relaxation. However, N-monomethyl L-arginine unmasked altered contractile sensitivity to norepinephrine in arteries from DOCA-salt hypertensive rats. These data show that the endothelium compensates for increased contractile and reduced relaxant responses of vascular muscle in DOCA-salt hypertension by increasing the release of endothelium-derived relaxing factor. These data suggest that altered vascular responsiveness is masked by the endothelium, thus preventing these alterations from contributing to increased peripheral resistance during the development of DOCA-salt hypertension.