Controversy continues as to whether hypotension during cardiopulmonary bypass (CPB) impairs intraoperative and postoperative renal function. Therefore, 21 patients with normal renal function (plasma creatinine less than 1.2 mg/dL, creatinine clearance greater than 70 mL/min), aged 50 to 70 years, without associated pathology, scheduled for elective coronary surgery were studied prospectively. Patients were randomized into two groups: group 1 included 14 patients whose arterial blood pressure during CPB was left untreated, and group 2 consisted of 7 patients who received phenylephrine to maintain their arterial pressure above 70 mmHg. Plasma and urine creatinine, sodium, potassium, and osmolality were measured preoperatively, intraoperatively and postoperatively. Creatinine, osmolal and free water clearances, and excreted sodium fraction were calculated. Plasma creatinine remained normal throughout the study in all patients. Creatinine clearances were similar preoperatively (101.9 +/- 36.7 in group 1 and 120.6 +/- 50.7 mL/min in group 2). In group 1, creatinine clearance decreased during CPB to 88.7 +/- 39.7 mL/min, whereas in group 2 it increased to 157.6 +/- 79.5 mL/min; the difference between groups was significant. Early postoperatively, there was no difference: 136.2 +/- 86.6 mL/min in group 1 and 100 +/- 21.4 mL/min in group 2. One week postoperatively, values were 100.5 +/- 37.9 and 101.9 +/- 18.4, respectively. There was a significant correlation between the creatinine clearance and perfusion pressure intraoperatively, but not postoperatively. Osmolal clearance also correlated with perfusion pressure intraoperatively, but it was significantly lower in the phenylephrine group postoperatively. Postoperative renal function was normal in all patients; no deleterious effect of a low arterial pressure during bypass could be identified.