Endothelin (ET) mechanisms were studied in hyper- and hypo-thyroid states in rats. Hyperthyroidism was induced by daily administration of thyroxine (0.1 mg/kg, i.p.) for 8 weeks, while hypothyroidism was induced by daily administration of methimazole (10 mg/kg, i.p.) for 8 weeks. The concentration of endogenous ET-1 was determined in the kidneys using radioimmunoassay. Systemic hemodynamics and renal blood circulation was measured using a radioactive microsphere technique. A significant increase in systolic and diastolic blood pressure, heart rate and cardiac output was observed in hyperthyroid rats as compared to eu- and hypo-thyroid rats. Total peripheral resistance was found to be similar in eu-, hyper- and hypo-thyroid rats. The endogenous concentration of ET-1 in the kidneys was significantly lower in hyper- as compared to eu- and hypo-thyroid rats. The blood flow to the kidneys was significantly increased in hyper- as compared to eu- and hypo-thyroid rats. Infusion of ET-1 (100 ng/kg/min i.v. for 45 min) produced a significant decrease in blood flow to the kidneys of eu-, hyper- and hypo-thyroid rats. The decrease in blood flow was similar in eu-, hyper- and hypo-thyroid rats, indicating that the response of renal blood vessels to exogenous ET-1 is not altered during thyroid dysfunction. Since endogenous ET-1 is involved in the regulation of vascular tone, it may be concluded that in hyper-thyroid rats decrease in concentration of the renal ET-1 could be contributing to an increase in blood flow to the kidney.