The effects were studied postoperatively of an infusion of atrial natriuretic peptide (ANP) 7.5 pMol.kg-1.min-1 on renal function and haemodynamics in seven patients who had been operated with insertion of an abdominal aortic graft. Urine flow, glomerular filtration rate (GFR), renal plasma flow (RPF) and excretion of electrolytes and osmoles were measured for three periods of 20 minutes during infusion of ANP, in the morning of the day after surgery. Haemodynamic studies were conducted, and serum levels of ANP, catecholamines and plasma renin activity were measured. ANP levels increased from 52 to approximately 250 pMol.L-1 during ANP infusion and decreased after infusion to a level equal to baseline. GFR increased from 92 mL.min-1 by 58, 20 and 21%, respectively. RPF was unchanged. Urine flow rate increased from 1.99 mL.min-1 by 81, 151 and 173%, respectively. Fractional clearances of sodium, chloride and osmoles were increased during the second and third ANP periods whereas fractional potassium clearance did not change during the study. There were no changes in catecholamine levels or plasma renin activity during the study. Heart rate, mean arterial pressure and calculated systemic and pulmonary vascular resistance did not change whereas reductions occurred in cardiac index, mean pulmonary artery pressure, pulmonary artery wedge pressure and mean right atrial pressure. We conclude that infusion of ANP also in the postoperative situation increases GFR, diuresis and sodium excretion.