In a group of six patients diagnosed as having unilateral renovascular hypertension due to fibromuscular dysplasia, inulin glomerular filtration rate, (GFR) and PAH renal plasma flow, (RPF) clearances, urine flow (V), urine sodium (UVNa), potassium (UVK), urinary excretion of prostaglandin E2 (UVPGE2), thromboxane B2 (UVTxB2), and 6-keto prostaglandin F1 alpha (UVPGF1 alpha) were measured in each kidney before and after the i.v. administration of furosemide (20 mg). The basal values of GFR, RPF, UVNa, UVPGE2, UVTxB2, and UV6-keto-PGF1 alpha were lower (p less than 0.01) in the stenotic kidney. Furosemide increased RPF 11% and 50%, GFR 25% and 62%, and V 142% and 280% in the contralateral and stenotic kidney respectively. The increase of UVNa was similar in the two kidneys. In the stenotic kidney, both UVPGE2 and UV6-keto-PGF1 alpha increased significantly (p less than 0.01) with furosemide while UVTxB2 remained unchanged. Furosemide did not alter the rate of excretion of the three prostaglandins measured in the contralateral kidney. We conclude that furosemide significantly improves renal circulatory and excretory function of the stenotic kidney. Since prostaglandin excretions also increased, the vasodilatation in the stenotic kidney may be prostaglandin mediated.