A male neonate, thus far healthy and fed with breast milk, developed rapidly increasing apathy on the third day of life. Sucking became poor and hyperhidrosis was present. Hyperammonemia (3,305 micrograms/dl) was noted. He became comatose and died on the fourth day. There was a profound derangement of amino acid concentration in the body fluids, with highly elevated citrulline levels (4.70 mumols/ml in serum and 8.47 mumols/ml in urine). Autopsy showed diffuse pulmonary bleeding, as the only noteworthy pathological finding. The liver contained no detectable argininosuccinate synthetase (ASS) activity. The defect of ASS in the present case was not due to a reduced amount of ASSmRNA, but its structure was found to be abnormal; it was approximately 1.57 kb in length due to a defect of about 0.1 kb near the 3' end of the coding region.