OBJECTIVE To determine the effects of synthetic human atrial natriuretic peptide (ANP) on renal function, hemodynamics, and levels of vasoactive peptides when infused in the immediate postoperative period after coronary bypass surgery in patients with normal kidney function. METHODS A prospective, randomized, double-blind, placebo-controlled study. METHODS The Department of Cardiothoracic Anaesthetics and Intensive Care of a university hospital. METHODS Thirty patients with normal kidney function scheduled for elective coronary bypass surgery. METHODS During the first 3 hours postoperatively, patients received an infusion of either ANP 7.5 pMol/kg/min (ANP) or vehicle alone (C). RESULTS No differences were found between the two groups in respect to sex, degree of coronary disease, preoperative medical treatment, or duration of cardiopulmonary bypass and aortic occlusion. Plasma ANP levels increased nearly 10-fold from a mean of 7.0 +/- 1.1 pMol/L in ANP and remained at baseline levels in C, (p < 0.001). In ANP, there occurred significant increases in urine flow (p < 0.001), inulin clearance (p < 0.001), filtration fraction (p = 0.007), and fractional clearance of sodium (p < 0.001) and of osmoles (p < 0.001) compared with C. During the study, no differences in mean arterial pressure, heart rate, and right atrial or pulmonary capillary wedge pressure were detected between the groups. Cardiac index decreased by 5% in ANP compared with a 9% increase in C (p = 0.027). Vasopressin levels significantly increased in C but remained at baseline levels in ANP (p = 0.031). There were no changes in levels of catecholamines or angiotensin II. CONCLUSIONS The results of this study show that ANP increases diuresis, natriuresis, and glomerular filtration in the immediate postoperative period after coronary bypass surgery.