Anuran amphibia which are not acclimable to high salt, tolerate high plasma urea.

1. The capacity of five anuran Amphibians (Bufo viridis, B. regularis, Rana ridibunda, Hyla arborea and Pelobates syriacus) to acclimate to NaCl and urea solutions was investigated. 2. All species could be acclimated to relatively high concentrations of urea solutions, while only Bufo viridis and Hyla arborea could be acclimated to 500 mOsm/kg or higher NaCl solutions. 3. The plasma urea concentration in B. viridis and H. arborea was elevated to levels over 140 mmol/l. 4. The sum of plasma sodium and chloride concentrations did not increase over 400 mmol/l in any species. 5. Urine osmolality, which was normally low, increased, but never exceeded the plasma osmolality. 6. In the urea acclimation conditions, urine electrolytes diminished, similarly in all species in this study. 7. It is concluded that anuran Amphibians can tolerate high plasma urea concentrations, but only those species which can elevate it, either through retention or net synthesis, can be acclimated to high salt solutions.