Cadmium, 10-3mol/l on the mucosal or 10-5 mol/l on the serosal side of the toad urinary bladder, inhibits the hydro osmotic effect of vasopressin. This inhibition is irreversible. The osmotic transfer of water in the absence of vasopressin was unaffected by the presence of the Cd2+. The hydro osmotic response to cyclic AMP was also reduced to the Cd2+, but the response due to hypertonicity of the serosal bathing solution was unaffected. The short circuit current (reflecting active transmural Na+ transport) was inhibited by 10-3mol Cd2+/l on the serosa, but was increased by 10-3 mol/l at the mucosa or 10-4 mol/l at the serosa. The natriferic response of the bladder to vasopressin was unaffected when Cd2+ was present under conditions that inhibited the hydro osmotic response, further emphasizing that separate effector mechanisms may be involved for each effect.