SUMMARY The results support previous findings that vasopressin produces dephosphorylation of a toad bladder membrane protein of 50000 mol. wt. At vasopressin concentrations of 50 mu./ml phosphorylation of this protein was 60 ± 7% of the control level, and maximal natriferic and hydro-osmotic responses were observed in intact bladders. However, at concentrations of vasopressin of 10 mu./ml when a maximal natriferic response but no significant hydro-osmotic response were observed, there was no significant difference in phosphorylation of the proteins from control or vasopressin-treated bladders. We conclude that the 50000 mol. wt membrane protein is likely to be associated with the hydro-osmotic response of the toad bladder to vasopressin.