Potassium balance was investigated in forty-five patients who underwent open-heart surgery. The patients were divided into a short-perfusion group (SPG, 24 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 andpostoperatively all patients were given furosemide when necessary, i.e. when the urine output declined below 0.5 ml/kg/h. In both the short- and the long-perfusion groups potassium balance was negative on the day of operation and the following day. On the day of operation the patients in the SPG with furosemide prophylaxis had a negative potassium balance of 116 mmol and without prophylaxis of 33 mmol. The corresponding values in the LPG were 35 and 31 mmol. On the first postoperative day the values were 33 mmol and 50 mmol in the SPG and 29 mmol and 36 mmol in the LPG, respectively. The serum potassium level declined slightly in all the groups; the greatest decline was recorded for the SPG patients with prophylactic furosemide administration.