Metabolic evolution of osmolarity, natraemia, chloremia, kaliemia and uremia of 22 infants with severe hypertonic dehydration, treated with fluidotherapy and peritoneal dialysis, is revised. Peritoneal dialysis was prepared in accordance with a special method for this metabolic condition. The serie was divided into two groups, according to initial value of natraemia: more or less than 170 mEq./l. Average decreases per hour of osmolarity, natraemia and chloremia were evaluated, as well as the influence of peritoneal dialysis on body fluids composition. Indication of peritoneal dialysis specially prepared for severe cases is discussed.