A 48-year-old man, who was diagnosed as adult-type citrullinemia by quantitative estimation of urea cycle enzymes in the liver, showed a regular nocturnal rise of blood ammonia level. In order to elucidate the mechanism of diurnal fluctuation of blood ammonia level, the patient was put into fasting state for four days. The blood ammonia level rose at the night of the first fasting day even though no food was taken, then it decreased gradually and reached the lowest level in the morning of the third fasting day. Intravenous administration of amino acids mixture under the same starved condition gave rise to a significant elevation of blood ammonia level. Based on these results, it was concluded that hyperammonemia of adult-type citrullinemia could result from the accumulation of free ammonia which was produced from the catabolism of amino acids absorbed from the small intestine and surpassed the urea synthesis of defective urea cycle to flood into the blood. Furthermore, the rise of blood ammonia level at the night of the first fasting day suggested that the circadian rhythm of amino acid-carbohydrate metabolism might superimpose on the process mentioned above.