It is well known that the intake of sulfate-containing natural mineral waters leads to contraction of the gallbladder, probably induced by the release of cholecystokinin (CCK). As early as 1959, there were some hints in the literature of circadian variations in gallbladder response; to find out whether this applies with sulfate as a stimulus, a pretest for basic information about gallbladder reaction to sulfate-containing mineral water was carried out on 19 healthy volunteers. On this basis, 15 healthy subjects of both sexes were then studied. After 6h of fasting, 500 mL of a sulfate-containing mineral water (2,800 mg SO4(2-)/L) were ingested within 5 min. The size of the gallbladder was registered ultrasonographically before and 15, 30, 60, and 120 min after drinking. The experiments were carried out seven times at different hours of the day for each volunteer. After the intake of the mineral water, the mean gallbladder size decreased significantly, followed by an increase after 60 min (P < .001). Significant circadian spontaneous variation in gallbladder size was detected (acrophase around 09:00; amplitude was 30.0% of daily average, P < .001). The contraction induced by the sulfate-containing water was most marked in the early morning hours and minimal around mid-day; the amplitude of this variation accounting for 29.0% of the daily average (P < .01). In contrast, the postdrinking relaxation was maximal around 18:00 and minimal around 9:00 (amplitude 38.5%. P < .001). These results show that the basal size of the gallbladder and its reaction to stimuli show a marked circadian variation: Whereas contractibility is maximal in the morning, dilatation is stronger in the afternoon.