The release of endothelin-1 from mesenteric arteries from cyclosporine-treated rats was measured by a specific enzyme immunoassay after purification of the perfusate on an immunoaffinity column. Mesenteric arteries from cyclosporine-treated rats (25 mg/kg per day for 6 weeks) released a significantly larger amount of endothelin-1 than arteries from vehicle-treated control rats (P less than 0.05). Serum creatinine levels were not significantly different in the two groups. These findings indicate that cyclosporine is a potential inducer of endothelin release from the mesenteric artery.