1. The osmotic and ionic regulatory abilities of adults of a freshwater population of the green toad (Bufo viridis) have been studied. These toads tolerated environmental salinities as high as 19% at temperatures near 25° C. Two individuals of another population of this species tolerated salinities as high as 23%

2. Changes in body weight of toads transferred to different environmental salinities indicate that the skin of this form is permeable to water. Rapid return to control levels of body weight indicate that drinking of external medium may be an important part of the initial adjustment to high salinities.

3. Above salinities of about 8% plasma Δ rises with increasing environmental Δ. Marked hypertonicity of the plasma is maintained in low salinities, but isotonicity with the medium is approached in higher salinities. Increases in plasma concentration above freshwater levels are due primarily to increased NaCl concentration (about 84%), partly to increased concentrations of urea (5-10%) and other osmotically active substances.

4. Urinary Δ is much lower than plasma Δ in dilute media, but becomes identical with plasma Δ above salinities of about 15%. Increases in urine concentration above freshwater levels are also due primarily to NaCl increase (74%). Considerable quantities of salt are lost via the urine. The kidneys seem to lose much of their ability to regulate urinary salt concentrations in high-salinity media.

5. Measurements of electrical potential and short-circuit current indicate that active uptake of inorganic ions by the skin continues in concentrated media, but at reduced rates.

6. Changes in muscle water, Na and K contents indicate the occurrence of some redistribution of water and salts between various body-fluid compartments as part of the salinity adaptation process.

7. In preference experiments, B. viridis chooses the land over any aquatic environment. Among aquatic environments it prefers those with salinities below 8%.

8. When combined with some earlier data by other workers who studied other populations of B. viridis, the present data indicate great uniformity of ionic and osmotic regulatory abilities among populations of this species. The marked differences between salinity tolerances of different populations are indicated to be due to differences in tissue tolerance of high body-fluid salinities.

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