Gallbladder filling was studied using computer-assisted cholescintigraphy in normal subjects who had fasted overnight. The gallbladder tended to visualize earlier than the distal part of the common bile duct. It appeared at approximately the same time regardless of whether or not there was passage of activity into the duodenum. This suggests that filling is not dependent on contraction of the sphincter of Oddi. Sequential images demonstrated that the activity entering the gallbladder rapidly reached the fundus. Time-activity curves showed a gradual buildup of activity in the bile ducts followed by sudden entrance into the gallbladder. Time-activity curves of the gallbladder's proximal and distal parts showed signs of an exchange of activity, suggesting that the gallbladder's motor function is not quiescent during fasting. Gallbladder motility could explain the periodic irregularities on the time-activity curve. These irregularities were smaller but not less frequent after morphine administration.