At the end of cardiopulmonary bypass (CPB) diuresis and natriuresis are widely modified. Those are classically due to the CPB conditions (mean arterial pressure, non pulsatile flow, hypothermia, long duration ...). Previous studies showed no evidence of these modifications being due to variations of hormones such as vasopressin, renin or aldosterone. The atrial natriuretic factor, cardiac hormone mainly known for its natriuretic effect, would contribute to explain these facts. This study includes 17 patients NYHA I or II without any renal dysfunction or diabetes mellitus. They were scheduled for cardiac surgery under CPB (valvular replacement or aortocoronary bypass). Sampling times were: TO: after induction of anaesthesia and before surgical incision; T1: during steady CPB; T2: 30 min after CPB release. At each time were obtained: diuresis, osmolar and free water clearance, fractional excretion of sodium, haematocrit, plasma concentration of ANF (pANF), and right atrial pressure and capillar wedge pressure in case of aortocoronary graft. FeNa at the end of CPB is significantly linked to the osmolar clearance and the CPB duration. FeNa evolution is parallel with pFAN evolution. At the end of CPB pFAN is first linked to cardiac rate, then to CPB duration. Cardiac filling pressures after and before CPB are not different. pANF after CPB cannot be attributed to these pressures. Numerous factors are involved in the renal sodium elimination. An evident statistic link between pANF and FeNa is then difficult to demonstrate. Their parallel evolution is coherent and suggests that ANF is the main hormone of natriuresis at the end of CPB. ANF secretion factors at the end of CPB remain unclear. This study emphasizes the involvement of cardiac rate and CPB duration in pANF increase at the end of CPB.