The vascular effects of isoproterenol and nitroprusside were evaluated in the perfused hind limbs of age-matched normotensive Wistar (NW) and spontaneously hypertensive (SH) rats. The vasodilating effects of these two compounds were assessed in the presence of concentrations of noradrenaline sufficient to raise the basal perfusion pressure to 85--110 mmHg. The sensitivity of the hind limb vessels of SH rats to the vasodilating action of isoproterenol (2.4 x 10-6--1.0 x 10-4 M) was slightly greater than that of the hind limb vessels of NW rats. On the other hand, the sensitivity to the vasodilating effect of nitroprusside (1.7 x 10-7--3.3 x 10-5 M) was similar in the hind limb vessels of both groups of rats. Propranolol (7.6 x 10-6 M) inhibited to a similar extent the isoproterenol-induced dilation of NW or SH hind limb vessels, but did not modify the vasodilating effect of nitroprusside in these preparations, thus suggesting that the vasodilating action of isoproterenol was due to the stimulation of vascular beta-adrenergic receptors. These results do not support the hypothesis suggesting the existence of a deficient vascular relaxation in hypertensive rats.