We studied in Xenopus laevis the effect of changing the salinity and the acid-base status of the ambient water on the total nitrogen catabolism and the nature of the nitrogen end products, urea and ammonia. Increase of the ambient osmolarity by addition of NaCl led to a rise in protein catabolism and to a predominant ureotelism which can approach 95% of the excreted nitrogen. The osmolarity can reach 500 mosmol. L-1 without obvious harmful effects. NaCl can then be replaced by NaHCO3 without injury to the animal as long as water alkalosis is avoided by an appropriate increase of the ambient CO2 tension, PCO2. However, if PCO2 is kept low, the resulting water metabolic alkalosis causes death within a few hours.