A chronobiological study was carried out in 8 elderly male subjects (74–85 years) to evaluate the 24-hour vasoactive intestinal polypeptide (VIP) secretory pattern. Eight young adult males (21–32 years) made up the control group. Blood samples were drawn from each subject every 2 h during the day and hourly during the night for a 24-hour period. Mean 24-hour VIP values in elderly (21.1 ± 0.3 pg/ml) and young adults (19.1 ± 0.3 pg/ml) did not differ statistically, but daytime VIP values observed in elderly subjects (21.4 ± 0.5 pg/ml) were higher (p < 0.05) than those recorded in young adults (17.5 ± 0.5 pg/ml). The young adults showed significant (p < 0.05) circadian VIP fluctuations with highest values during the nighttime, while the elderly subjects did not. An age-related decreased activity of the peripheral neuronal VIP-ergic network is hypot