To investigate the diurnal variation in the plasma atrial natriuretic peptide (ANP) concentrations and its relation to renin activity, aldosterone, urine volume and electrolytes, blood samples from six healthy male subjects were collected and blood pressure and heart rate recorded every 4 h during a 24-h sleep-wake period. Systolic blood pressure and heart rate were at the lowest during sleep. Plasma ANP, extracted on Sep-Pak cartridges and measured by radioimmunoassay, had the highest concentration at midnight and lowest at 04.00 h (27 +/- 18 vs 16 +/- 18 ng l-1, mean +/- SD, P less than 0.05 after normalization of the original values). Plasma renin activity and aldosterone were at the highest at 08.00 and 12.00 h, whereafter they both began to decrease (P less than 0.001, also normalized). Urinary volume, sodium, potassium and chloride showed highest values (P less than 0.05) between 08.00 and 16.00 h. Plasma ANP levels did not correlate with systolic blood pressure, heart rate, serum or urinary Na or plasma renin or aldosterone (r = 0.1-0.3, P greater than 0.05). Plasma renin activity correlated with aldosterone and aldosterone with urinary K (r = 0.66 and 0.58 respectively, P less than 0.001). These results suggest that the low plasma ANP levels found in our subjects at 04.00 h may be associated to diminished atrial distension accompanied by the inactivity of the sleep period. In addition, ANP does not appear to affect the secretion of renin and aldosterone in a normal sleep-wake cycle.