The effect of changes of dietary sodium chloride intake and posture on plasma atrial natriuretic peptide concentration and renal function was studied in 11 normal human volunteers. Plasma atrial natriuretic peptide concentration was higher in the upright posture on a high than it was on a medium or low salt diet. On the medium and high but not on the low salt diet the concentration increased significantly on adoption of the supine posture. Creatinine, sodium, lithium and fractional lithium clearances, fractional distal sodium excretion and total distal water and sodium reabsorption, which were estimated by the lithium clearance technique, were significantly higher on the high than on the low salt diet. The medium salt intake gave intermediate values. Heart rate while upright was significantly higher on the low than on either the medium or the high salt diets. Systolic blood pressure was unaffected by salt intake. Diastolic blood pressure in the supine position was significantly higher on the low than on the medium or high salt diets. Both plasma noradrenaline concentrations and plasma renin activity were significantly higher on the low than on the high salt diet. Values on the medium salt intake were intermediate. Plasma concentrations of both hormones were higher in the upright than in the supine posture on all three salt intakes. The data are consistent with the hypothesis that atrial natriuretic peptide contributes to the cardiovascular and renal adjustments to changes in dietary sodium chloride, and the possible role of the peptide is discussed.