Abstract 1. 1. A reduction in osmotic permeability to water and an increase in sodium and chloride absorption were observed in the isolated urinary bladder of three species of flounders after their acclimation to hypotonic media and after prolactin treatment. 2. 2. Bladders of stenohaline fresh-water fishes were nearly impermeable to water; active absorption of sodium was indicated in some species. 3. 3. Osmotic permeability was variable in marine fishes, but was especially noticeable in aglomerular Porichthys and Lophius. 4. 4. Euryhaline fishes of sea-water origin modify their bladders from a permeable state in sea water to a relatively impermeable state in fresh water. Prolactin seems to be involved in this modification. Bladders from euryhaline fish of fresh-water origin appear impermeable, irrespective of environmental salinity.