Immunoreactive atrial natriuretic factor (ANF) levels were measured preoperatively and for 5 days postoperatively in 22 patients undergoing cardiothoracic surgery. They were studied in 4 groups according to surgical procedure (mitral valve replacement, aortic valve replacement, coronary artery bypass grafting, and lung resection). The highest preoperative ANF levels were observed in the aortic valve group (mean 40.9 pmol/L) which were 2.5 (95% CI: 0.7 to 8.6) to 3.5 (95% CI: 0.9 to 13.9) times higher than the other groups. Values tended to peak on the 3rd and 4th postoperative days in all groups, although significantly elevated postoperative ANF concentrations occurred only in the coronary artery bypass group where the levels increased by a factor of 3.2 (95% CI: 1.3 to 7.5). As a separate part of the study, measurement of ANF release before, during, and after cardiopulmonary bypass in 6 patients tended to show a fall in ANF levels when on bypass, with a return to baseline levels on cessation of bypass.