The effects of hyper- and hypothyroidism on the vasorelaxing responses to acetylcholine (ACh), sodium nitroprusside (NP), and CaCl2 were investigated in aortic strips and isolated perfused kidneys. The renal vascular reactivity to ACh and NP was increased in hyperthyroid rats, whereas the concentration-response curve to ACh in hypothyroid rats was flattened. In the renal vasculature from hypothyroid rats, NP produced a dual response: vasoconstriction at low doses and vasodilation at medium to high doses. Aortic strips from hyperthyroid rats showed an increased response to ACh without significant differences between hypothyroid and control groups. Aortic strips from all three experimental groups showed a similar relaxing response to CaCl2. These results indicate that: (1) the raised arterial pressure of hyperthyroid rats is not associated with a reduced endothelium-dependent and calcium-induced vasodilation, and (2) the changes in responsiveness to vasodilators in resistance vessels from hyper- and hypothyroid rats may play a role in the increased and decreased peripheral vascular resistances, respectively, previously reported in such animals.