1. Total rates of urea loss from adult euryhaline crab-eating frogs (Rana cancrivora) adapted to various environmental salinities between fresh water and 80 % sea water increase as salinity increases above 40% sea water. Oxygen consumption is constant in rate in all salinities studied.
2. The presence of urea in the Ringer solution bathing isolated pieces of skin of frogs adapted to 60% sea water increases both the electrical potential and the inwardly directed short-circuit current across the skin.
3. In skeletal muscle cells addition of intracellular solutes maintains tissue hydration in the face of large increases in plasma osmotic concentration in high-salinity media. Changes in the intracellular urea and free amino acid concentrations are primarily responsible for increases in intracellular osmotic concentration.
4. Some implications of these observations are discussed and comparisons made with the euryhaline green toad, Bufo viridis.