1. An in vitro preparation of the urinary bladder of Bufo marinus is described.
2. Small doses of 'Pituitrin' markedly increase the rate of water transfer across the bladder wall when the solutions inside the bladder are hypotonic.
3. Passive movement is small and increases slightly with increases in the osmotic gradient across the bladder wall. It is unaffected by changes in substrate levels or any of the metabolic inhibitors tested except for cyanide which increases it in some cases.
4. The vasopressor neurohypophysial fraction is more active than the oxytocic one in increasing water transfer across the bladder wall.
5. The increase in water transfer depends on an intact oxygen supply and sufficient glucose or pyruvate.
6. Iodoacetate, malonate, cyanide, 2–4-dinitrophenol, and bubbling 5% CO2+95% O2 through Ringer's solution inhibit the water transfer in response to neurohypophysial extract.
7. Diamox is only an effective inhibitor at very high concentrations.
8. The possible mechanism of the water transfer is discussed.