The effect of furosemide on renal function in patients undergoing open heart surgery was investigated, using creatinine clearance, urine flow, sodium and potassium excretion as the parameters. The effect of furosemide (2 mg/kg i.v.) on blood flow in arteria renalis and the tissue oxygen tension of the renal cortex and medulla were also investigated experimentally in six dogs. The flow was measured with an electromagnetic flowmeter and the tissue oxygen tension with IBC tissue oxygen electrodes. Prolongation of the perfusion time to more than 60 minutes resulted in a decline of creatinine clearance to appr. 50% of the initial level. Prophylactic furosemide given prior to the perfusion brought down the creatinine clearance to 64% of the initial level. Furosemide, administered either prophylactically just before the cardiopulmonary bypass or after urine flow had declined below 0.5 ml/kg/h, had no effect on the potassium balance in the long perfusion group (over 60 minutes), though it clearly increased the negative potassium balance in the short perfusion group (below 60 minutes). In an experimental work on dogs, furosemide was found to elevate the tissue oxygen tension in the renal cortex and and medulla, and slightly to increase the blood flow in arteria renalis.