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1. The capacity of adaptation of toads (Bufo bufo) to environments of high salinity was studied and the relative importance of skin, kidney and urinary bladder in controlling the balance of water and salt was assessed.

2. Toads were kept in NaCl solutions of 20, 50, 110, 150 and 220 mM and studied in their fourth week of adaptation. A group of animals considered as `control' was kept in wet soil with free access to water. Plasma, ureter urine, and bladder and colon contents were analysed for sodium, potassium, chloride and osmolality, and total body sodium and water were determined. Absorption of water and <sup>22</sup>Na through the skin, and water flow and sodium excretion through the ureter, of intact animals was studied. Hydrosmotic water transport through the isolated urinary bladder of `control' and adapted animals was determined. The effects of pitressin and aldosterone on the water and sodium balance are described.

3. The survival rates of toads kept in saline concentrations up to 150 mM were identical to that of `control' animals, but half of the animals kept in 220 mM died within 4 weeks.

4. There is a linear correlation between the sodium concentrations and osmolality of plasma and of the external media.

5. The sodium concentration in colon contents rose with rising external concentrations, up to values higher than the values in plasma.

6. Sodium concentrations and osmolalities of ureter and bladder urine increased in adapted animals, the values for bladder urine becoming much higher than those for ureter urine in animals adapted to 110, 150 and 220 mM.

7. Total body water, as a percentage of total weight was kept within very narrow limits, although the total body sodium increased with adaptation.

8. Absorption of water through the skin for the same osmotic gradients was smaller in adapted than in `control' animals.

9. The ureteral output of water of toads adapted to 110 and 150 mM-NaCl was larger than the water absorption through the skin.

10. Skin absorption of sodium was lower in animals adapted to concentrated saline solutions than in `control' animals.

11. Sodium output by the ureter was identical to skin absorption in `control' animals adapted to 20, 50 and 110 mM-NaCl but was higher in animals adapted to 150 mM-NaCl.

12. Aldosterone increased the absorption of sodium in `control' and adapted toads, but at all dose levels absorption by control was greater than by adapted animals.

13. The stimulation of water absorption by vasopressin in vivo or in isolated bladders was not modified in animals adapted to high salinities.