1. 1. Dehydration of toads with empty bladders causes elevation of the sodium concentration (C) of their plasma according to the relationship: , where C0 is the plasma sodium concentration of the hydrated toad and volumes are expressed as ml/100 g of hydrated toad.2. 2. The concentration of potassium in the plasma declines in the initial stages of dehydration and returns to the hydrated level only when the toad has lost 20-30 ml of water/100 g.3. 3. The sum of the concentrations of sodium and potassium in tissue water is increased in dehydrated toads according to the same relationship found for plasma sodium.4. 4. Tissues lose a smaller fraction of their water than does the whole toad during dehydration. The retention of water by tissues during dehydration is apparently associated with their uptake of electrolytes.5. 5. Dilute urine stored in the bladder may be utilized by toads during dehydration in maintaining the sodium concentration of the plasma at the normal level. As water is lost in excess of the volume of urine initially present in the bladder, the plasma sodium concentration increases as described in (1) above.