The permeability barrier to water of the toad's urinary bladder can be locked in various stages of response to neurohypophyseal hormones by reacting the bladder mucosa with a 1% glutaraldehyde solution for 5 min. In vitro experiments have shown that bladders which have been exposed to oxytocin, fixed, and removed to hormone—free Ringers fluid still retain 78% of their pre—fixation permeability to water. With this technique the hydro—osmotic response of the bladder to vasotocin has been followed in the absence of net transmembrane water flux over a time interval ranging from 5 sec to 20 hr. In order to measure changes in the permeability to water of the bladder in situ, toads were either injected with vasopressin or progressively dehydrated, and they were then pithed and their urinary bladders immediately removed and fixed. The hydraulic conductivity of the fixed bladder was then measured under standard conditions in a tissue bath. A half—maximal permeability response of the bladder was obtained with injection of 10 mU vasopressin/g bw into the dorsal lymph sac. In dehydration experiments the hydraulic conductivity of the bladder remained low in animals with plasma osmolalities below approximately 300 mOsm/1, but it increased suddenly and by as much as 60–fold as the plasma osmolality exceeded 300 mOsm/1. (Endocrinology91: 240, 1972)