Atrial natriuretic peptide (ANP) shows a nychtohemeral fluctuation and an age-related trend. The aim of this study was to explore the circadian rhythm of ANP as a function of age. Circadian rhythms of plasma renin activity (PRA), aldosterone (PA), and cortisol (PC) were explored as well. Twenty clinically healthy subjects, 10 young (20-25 yrs) and 10 elderly (65-75 yrs), were investigated, while recumbent, after synchronization to light-dark regimen and meal timing. Blood samples for RIA tests were collected six times during the 24-hr span. The chronobiologic analysis in young subjects demonstrated a significant circadian rhythm for all the investigated variables with an acrophase-timing located at 16.48 for ANP, 4.44 for PRA, 5.32 for PA, and 7.12 for PC. In elderly subjects we documented an important increase of 24-hr mean plasma levels but not a statistically significant circadian rhythm for ANP, and a decrease in mean value of PRA which maintained, however, a significant periodic 24-hr oscillation in parallel with PA and PC. The results in young subjects reinforce the concept that ANP plays physiologically an inhibitory role on the phasic secretion of renin. The lack of the circadian rhythm for ANP along with the divergent changes in ANP and PRA 24-hr mean concentration of elderly subjects both suggest that ANP exerts with advancing age only a counterregulatory role on the tonic rather than the phasic release of renin.