1. The osmotic and ionic regulatory abilities of adults of the euryhaline crab-eating frog (Rana cancrivora) have been studied. Adult frogs tolerated environmental salinities as high as 28‰ at 30°C. Tadpoles of this form tolerated salinities as high as 39%‰ at the same temperature.

2. Changes in body weight of frogs following transfers to different environmental salinities indicate both that the skin of this frog is permeable to water and that these animals do not swallow large volumes of external medium, even in high salinities.

3. Above salinities of about 9%‰, plasma Δ rises with increasing environmental Δ. Plasma Δ is always higher than environmental Δ. Increases in plasma concentration above fresh-water levels are due partly to increased NaCl concentration (about 40%), partly to increased urea concentration (about 60%). Urea concentrations as high as 0.48 M (2.9%) have been measured.

4. Urinary Δ parallels plasma Δ, but is always lower than plasma Δ. Considerable quantities of urea are lost via the urine, even though urinary urea levels are below plasma levels.

5. Measurements of short-circuit current indicate that active uptake by the skin of inorganic ions continues in R. cancrivora acclimatized to high salinities.

6. R. cancrivora is no less susceptible to water loss by evaporation from the skin than are other amphibians.

7. In preference experiments R. cancrivora chooses salinities below 18%‰, but shows no strong preference for a particular salinity.

8. Similar observations on osmoregulatory mechanisms in a close relative of R. cancrivora, the tiger frog (R. tigerina), show that the latter species is similar to ordinary fresh-water frogs.

9. The striking physiological convergence between R. cancrivora and the elasmo-branch fishes is discussed, as are various possible implications of our data regarding nitrogen metabolism in tadpoles and kidney function in adult frogs.

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