A simple and reproducible method has been developed for following fluid transport by an in vitro preparation of mammalian gall bladder, based upon weighing the organ at 5 minute intervals. Both guinea pig and rabbit gall bladders transport NaCl and water in isotonic proportions from lumen to serosa. In the rabbit bicarbonate stimulates transport, but there is no need for exogenous glucose. The transport rate is not affected by removal of potassium from the bathing solutions. Albumin causes a transient weight loss from the gall bladder wall, apparently by making the serosal smooth muscle fibers contract. Active NaCl transport can carry water against osmotic gradients of up to two atmospheres. Under passive conditions water may also move against its activity gradient in the presence of a permeating solute. The significance of water movement against osmotic gradients during active solute transport is discussed.

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