SUMMARY: Blood plasma of dehydrated toads (Bufo marinus) and bullfrogs (Rana catesbeiana) increased water transfer (hydro-osmotic response) across the isolated toad urinary bladder. Sodium transport (in terms of the shortcircuit current) was also increased. Immersion of Bufo marinus, Rana catesbeiana and Xenopus laevis (but not the neotenous newt Necturus maculosus) for 2 hr. in a 2% NaCl solution also increased the hydro-osmotic activity of plasma. Ether and haemorrhage also had this effect on plasma from Rana and Bufo. Hydro-osmotic activity of plasma from dehydrated bullfrogs was destroyed by incubation with trypsin or tyrosinase, evidence which combined with the pharmacological effects of the plasma indicate that the hydroosmotic activity in the plasma is due to arginine vasotocin. The concentrations of this hormone were comparable with levels of antidiuretic activity observed in mammalian plasma.