Atrial extracts have long been known to produce natriuresis but it is only recently that atrial natriuretic peptide (ANP) itself has been isolated, purified and sequenced. We have now developed a specific radio-immunoassay for ANP and assessed its biological activity by infusing ANP in normal volunteers. Low dose ANP infusion (1.5 pmol/kg per min) produced no haemodynamic or natriuretic effects. High dose ANP infusion (15 pmol/kg per min) produced an increase in plasma immunoreactive ANP levels of 203 +/- 78 pmol/l and caused the urinary sodium excretion to increase from 3.5 +/- 1.6 to 11.0 +/- 7.4 mmol/30 min period (P < 0.05). No haemodynamic effects were seen with this high dose infusion. No changes in urinary dopamine were seen with either ANP infusion. This study shows that ANP is capable of producing a natriuresis in man in the absence of any generalized haemodynamic effects and that ANP may be an important modulator of salt and water balance in man.