The occurrence and extent of a circadian rhythm in the circulating concentrations of atrial natriuretic peptide (ANP) is still a matter of controversy. In a group of hospitalized normal volunteers (6 men and 4 women, 16–76 years old), we investigated the circadian variability of ANP and its temporal relation with the circadian rhythms of blood pressure (BP) and heart rate (HR), by using a chronobiological inferential statistical method. At the end of a synchronizing period of 1 week (the diet and the daily schedule were standardized), the subjects underwent automatic BP and HR monitoring, and blood sampling for 24 h. A statistically significant mean circadian rhythm was demonstrated for ANP, BP, and HR. The mean circadian acrophase of ANP was calculated to occur around 4 a.m. BP and HR rhythms appeared to be in antiphase with ANP rhythm, i.e. the peak of BP and HR rhythms more or less coincided with a trough in ANP rhythm. ANP appears to be anticipatory in its circadian periodic rise to awakening. Therefore, postural changes cannot fully account for the diurnal variations observed.