This study analyzed the role of NO and endothelium-derived hyperpolarizing factor (EDHF) in the abnormal renal vascular reactivity of hypothyroid rats. Renal responses to vasoconstrictors (VC: phenylephrine, PHE, and angiotensin II, AII) and vasodilators (VD: acetylcholine, ACH, nitroprusside, NP, and papaverine, PV) were studied in kidneys from control and hypothyroid rats, under normal conditions and after NO or EDHF blockade. NO was blocked by the administration of L-NAME, and EDHF by the administration of TEA or by an increased extracellular K⁺. The response to VC was also evaluated after endothelium removal. Hypothyroid kidneys showed reduced responsiveness to PHE and a normal response to AII. L-NAME and TEA administration produced an increased sensitivity to PHE and to AII in control preparations. L-NAME also increased the response to PHE in hypothyroid kidneys but the differences between control and hypothyroid kidneys were maintained. TEA administration did not change the response to either VC in hypothyroid preparations. In endothelium-removed preparations, TEA was unable to increase pressor responsiveness to VC. Hypothyroid kidneys showed reduced responsiveness to ACH and NP and normal response to PV. The differences between hypothyroid and control preparations in the responses to ACH and NP were maintained after L-NAME or increased K⁺. In conclusion, this study shows that: a) the attenuated response to PHE in hypothyroidism is not related to an increased production of endothelium-derived relaxing factors NO and EDHF; b) the response to VC in hypothyroid preparations is insensitive to EDHF blockade; and c) hypothyroid preparations have a reduced reactivity to the NO donor, and NO-independent vasodilatation remains unaffected.