Patients undergoing open-heart surgery were divided into two groups according to cardiopulmonary bypass time: in Group I the bypass time was less than 60 minutes (25 patients) and in Group II more than 60 minutes (25 patients). Renal function parameters were investigated preoperatively, during the operation and for two days-postoperatively. The urine volume and excretion of sodium and potassium increased significantly in both groups during the bypass period compared to the values of the pre-bypass period. Significant differences (p less than 0.001) were seen during the bypass period in free water clearance: -0,8 +/- 0.6 ml/min in Group I and -0.2 +/- 0.2 ml/min in Group II, and in creatinine clearance: 114 +/- 42 ml/min in Group I and 60 +/- 12 ml/min in Group II. The inter-group differences in free water and creatinine clearances were significant (p less than 0.001) from the bypass period to the second day period. In Group II, urine osmolality was significantly lower in serum osmolality significantly higher from the bypass period to the third day period compared to the values of Group I. The excretion of sodium and potassium was higher in Group I during bypass and postoperatively and this was reflected in the serum values of sodium and potassium. According to our results, signs of functional renal impairment were seen in the group with the longer perfusion. In this group, however, the patients were older and had a higher preoperative serum creatinine mean value than the patients with the shorter perfusion. The mechanism of functional renal impairment is discussed.