Deuterium and O18 were used simultaneously as tracers for water. It was found that in the moderately distended dog bladder a considerable exchange of water molecules takes place across the mucosal wall, but net volume changes are so small as to be difficult to demonstrate. In these expts. the average turnover rate of water in the bladder was 0.4%/min., and the rate of water exchange across the bladder mucosa was estimated to be of the order of 4 cu. mm./sq. cm./min.