Immunoreactive atrial natriuretic factor (ANF) levels were measured in blood taken from superior vena cava, right atrium, pulmonary artery, pulmonary vein, left atrium, and radial arteries in patients undergoing cardiac surgery. Significant (p less than 0.05) differences were seen between sites. Levels of atrial natriuretic factor were seen to rise from superior vena cava (27.5 pmol/L) to right atrium (54.3 pmol/L) and then fall in pulmonary artery (38.5 pmol/L). A further significant fall was seen in pulmonary vein (32.7 pmol/L) samples. There was no significant change in levels in left atrium (32.1 pmol/L) or radial artery (30 pmol/L). The fall between radial artery levels and superior vena caval levels was also significant. The rise from superior vena cava to right atrium is to be expected as this represents the major site of addition of atrial natriuretic factor to the circulation. We propose that the fall in levels from right atrium to pulmonary artery and from pulmonary artery to pulmonary vein suggests uptake and therefore possible local hormonal action on both right ventricle and pulmonary vasculature. The fall from radial artery to vena cava would be in keeping with atrial natriuretic factor's known systemic uptake and action.