Our major aims in this study were to determine the possible existence of ultradian rhythms related to migrating motor complex (MMC), as well as to characterize the existence of circadian rhythms in gallbladder bile concentration in fasting and feeding dogs. Conscious dogs with chronic indwelling gallbladder catheter to allow the periodic collection of bile by means of an automatized device, were used to determine if the composition of gallbladder bile shows rhythmic changes. During feeding, gallbladder bile concentration shows circadian rhythms peaking immediately before meal time in all biliary compounds. On the other hand, during interdigestive state an ultradian rhythm of periodic dilution (90 min period) of gallbladder bile during the bile concentrating process is observed.