24-hour basal gastric acid collections were obtained from 10 healthy, young (ages 22–27) male volunteers. The hourly secretory pattern, which described a circadian periodicity (low morning, high evening acid rates), was compared to mean plasma gastrin levels obtained at 4-hour intervals. No rhythm in plasma gastrin was detected over the 24-hour collection period. Hence, the circadian rhythm of basal acid secretion in man does not appear to be related to fluctuations in plasma gastrin levels.