Gallbladder volume and interdigestive gastric and duodenal motor activity were evaluated simultaneously in 12 normal subjects. After overnight fasting, gallbladder volume was monitored every 4 min in each subject by means of real-time ultrasonography, and gastroduodenal motor activity was measured by means of a probe consisting of three polyvinyl catheters with one side opening for each catheter, placed 15 cm apart and constantly perfused with deionized water. Real-time ultrasonography and intestinal manometry were performed by different investigators and continued until at least two consecutive spontaneous phase III activities of migrating motor complexes were observed. The results show a cyclic variation of gallbladder volume, which reached its minimum value before the end of phase II in the proximal duodenum and its maximum in early phase II, 25 min after the beginning of phase III. These results suggest that there is a relationship between the cyclic gallbladder volume changes, which occur during fasting in humans, and with the various phases of duodenal migrating motor complex.