Carbamyl phosphate synthetase (CPS) catalyses the synthesis of carbamyl-phosphate from ammonia and bicarbonate and is the first step in ureagenesis. The infant described in this report suffered from deficiency of this enzyme. The symptoms started on the 2nd day of life with tachycardia, apathy, irritability and metabolic alcalosis, on the 4th day coma and fits occurred due to hyperammonia (ammonia in the blood max 496 mumol/l, normally up to 150 in newborns). In hepatic tissue no activity of carbamyl phosphate synthetase could be measured (normal range 0.66-2.1 mumol/h/mg protein). Peritoneal dialysis was instituted, but the metabolic crisis could only be overcome by the following therapeutic measures: restriction of protein intake to 1.5 g/kg/d in part as a special aminoacid mixture, in part as breast milk; sufficient caloric supply (600-500 kJ/kg/d); sodium benzoate 350 mg/kg/d: arginine 2 mmol/kg/d respectively citrulline 350 mg/kg/d, and carnitine 150 mg/kg/d. By these procedures the exogenous and endogenous load of ammonia could be minimized. Electroencephalogram and mental development were normal. Acute metabolic crises with hyperammonia during catabolic states (infections) could be treated several times. At the age of 8 months, however, the patient died during such a crisis. This case shows that it is possible to achieve a normal psychomotor development in complete CPS-deficiency by adequate therapy. Catabolic states are difficult to manage.