Synopsis Ion and water movements in the isolated gall bladder of several teleost species were studied in relation to osmoregulation and to hormone treatments. When gall bladders were bathed on both mucosal and serosal sides with isosmotic Ringer solution, a considerable amount of water moved from mucosa to serosa in all the species examined, and sodium and chloride concentrations in the passing fluid were greater than those of the Ringer solution, indicating active transport of these ions. The

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... all bladder of the freshwater-adapted eel was identical in its ion and water absorption capacity with that of the eel in sea water. No change in this absorption capacity was observed after cortisol treatment of the freshwater eel or prolactin treatment of the seawater eel, although these treatments produced significant effects on ion and water movements in intestinal segments isolated from the same fish. Treatment of gall bladders from starry flounder with oxytocin and arginine vasotocin in vitro showed an inhibitory effect on water movement. Comparative studies of several teleost species reveal no correlation between marine or freshwater existence and the absorption capacity of the gall bladder, nor any relation of the latter to the structure of the gall bladder (tubular or sac-like). The lack of response to environmental salinity changes and to cortisol and prolactin indicates that the teleost gall bladder is not merely an ontogenic extension of the intestinal surface in its osmoregulatory ability.