A new instrument for automatic continuous sampling of bile has been developed. Bile produced by an animal is externally diverted into a reservoir. A detection device permits evacuation of the reservoir into separate chambers, so that a preset continuous sample of total flow is diverted for collection, while the remainder is returned to the animal. The system is closed to air. Biliary tract pressure may be regulated, and bile flow calculated. The apparatus has been found to be very reliable and accurate in over 1 year of use. This instrument is adaptable to sampling other biological liquids.