1. The effects of low dose infusion of atrial natriuretic peptide (ANP) were observed in double-blind, placebo-controlled study in six fluid-loaded volunteers. After baseline observations, hourly increments of 0.4, 2 and 10 pmol min-1 kg-1 were infused with continuous observation of heart rate, blood pressure and cardiac output. Plasma ANP, aldosterone, and catecholamines, and urinary volume and sodium excretion, were estimated at half-hourly intervals. 2. ANP infusion resulted in an increase of 35, 98 and 207% in urinary sodium excretion and of 10, 20 and 71% in urinary volume when compared with placebo. Plasma ANP was markedly elevated above placebo levels only during infusion of 10 pmol of ANP min-1 kg-1. 3. No change in heart rate of blood pressure was noted during the study, but a significant fall in stroke volume index was observed during active treatment. Plasma levels of aldosterone and catecholamines were not significantly different on the 2 treatment days. 4. The potent natriuretic and diuretic effects of this peptide at plasma concentrations not significantly elevated from physiological suggest a hormonal role for ANP in the homoeostasis of salt and water balance.