1. The effect of changes of posture on plasma atrial natriuretic peptide concentrations and renal function was studied in normal human volunteers.
2. Plasma atrial natriuretic peptide concentrations increased in the supine posture, reached a maximum value after 30–60 min, remained elevated for 4 h and decreased to baseline values on return to the upright posture. Inflation of antishock trousers, which apply positive pressure to the legs and lower abdomen, attenuated the fall in plasma atrial natriuretic peptide concentration in the upright position.
3. In the supine posture there were increases in urine flow rate, sodium, lithium, fractional sodium and fractional lithium clearances. Fractional distal water and sodium excretion, and total distal water and sodium reabsorption, which were estimated by the lithium clearance technique, also increased.
4. Heart rate and systolic and diastolic blood pressures decreased in the supine and increased on return to the upright posture. Inflation of antishock trousers prevented the increase in heart rate in the upright posture.
5. The contribution of haemodynamic factors to the increase in plasma atrial natriuretic peptide concentrations in the supine position and the relationship between this increase and the associated changes in renal function are discussed. However, the contribution of atrial natriuretic peptide to these changes is uncertain.