The gallbladder is conventionally regarded as an absorptive organ such that dilute hepatic bile is both stored and concentrated. We studied 35 patients who had recovered from a percutaneous transhepatic gallbladder drainage performed for acute cholecystitis. After an overnight fast, gallbladder bile was dark brown in color and had a wide scatter in the lipid composition. Two hours after a meal, the gallbladder bile was opalescent white in color and had the composition of an extracellular fluid. This phenomenon was uniformly observed in all 35 patients and was also consistently reproducible when five patients were repeatedly studied. We used normal dog gallbladder epithelial cell monolayers grown in culture and examined sodium flux. Control gallbladder cells absorbed sodium. When secretin (0.5-2.5 x 10(-7) M) was added, there was a prompt reversal of sodium flux, resulting in net secretion. We conclude that secretion is a physiological function of the gallbladder mucosa. After feeding, the neural and humoral factors divert stored and newly secreted bile into the duodenum and induce active de novo secretion thus producing a gallbladder bile that is opalescent white with no lipids. Our results also have important implications on the origin of the pathological "white bile," the pathogenesis and treatment of gallbladder sludge, as well as the kinetic analysis of compounds undergoing enterohepatic recirculation.