(1) Synthetic oxytocin (100 m-u./ml.) produces swelling of the isolated frog urinary bladder even in the absence of an osmotic gradient across the bladder.
(2) Calculations show that the change in intracellular space does not necessarily differ significantly from that in the presence of an osmotic gradient since the inulin space is markedly affected by the osmotic water flow.
(3) Part of the cellular potassium is exchanged for sodium during the swelling produced by oxytocin.
(4) A possible mechanism and the significance of the swelling is discussed.