The effect of recumbent rest on plasma atrial natriuretic peptide (ANP) concentration, mean blood pressure (MBP) and heart rate was studied in 26 normal human volunteers. Plasma ANP concentration, MBP and heart rate were determined after 10 min sitting and after 2, 5, 8, 15 and 30 min of recumbency. During the first 5 min of recumbency there were significant decreases in both MBP (p less than 0.001) and heart rate (p less than 0.001) compared with sitting. There was a small but significant fall in plasma ANP concentration (p = 0.02) after 30 min of recumbency compared with sitting. No further reduction in MBP or heart rate occurred after 5 min supine. The level of MBP following 5 min supine correlated significantly (r = 0.44; p = 0.02) with the plasma ANP concentration 25 min later. Blood pressure and heart rate are highly dependent on posture and relaxation, and plasma ANP concentration in lesser degree. It is necessary to wait for stable baseline values to develop before any comparisons between blood pressure and plasma ANP are done. If it is assumed that a causative relation exists between blood pressure and plasma ANP level, it appears there may be a delay of 20-30 min between a change in blood pressure and the physiological response of plasma ANP level. This observation may have implications for the interpretation of the relation between blood pressure and plasma ANP concentration in healthy individuals.