Forty-five patients who underwent open heart surgery were divided into a short-perfusion group (SPG, 21 patients) with a perfusion time shorter than 60 minutes and a long-perfusion group (LPG, 24 patients) with a perfusion time longer than 60 minutes. Nine patients in the SPG and 13 in the LPG received furosemide prophylactically prior to the perfusion. The furosemide dose was 20-60 mg. During the perfusion and postoperatively all patients were given furosemide when necessary, i.e. when the volume for diuresis per hour declined below 40 ml/h. The patients who received prophylactic furosemide in the LPG subsequently had clearly less need for furosemide (3.9 mg/h) than the control subjects (7.9 mg/h/3 day). The patients with furosemide prophylaxis in the LPG had significantly ( less than 0.05) higher creatinine clearance and lower serum creatinine values during the postoperative night period and on the 3rd day. In perfusions lasting less than 60 minutes the patients with furosemide prophylaxis had significantly higher urine flow (p less than 0.001), sodium excretion (p less than 0.001) and potassium excretio-n (p less than 0.01) during bypass surgery and postoperatively compared with that of controls. According to our findings, the prophylactic use of furosemide had a beneficial effect on glomerular filtration rate (endogenous creatinine clearance) and postoperative serum creatinine level in LPG. In perfusion shorter than 60 minutes furosemide prophylaxis may be harmful owing to the increased excretion of water, sodium and potassium.