A 29-year-old man who had a taste for peanuts frequently showed abnormal behaviors. Physical examination revealed consciousness disturbance associated with flapping tremor, hypotonus in the extremities, and hypoactive deep tendon reflexes. Laboratory examination showed hyperammonemia with mild liver injury. A level of serum citrulline was 14 times higher than the upper limit of normal. A diagnosis of type 2 citrullinemia (Seheki et al) was established by the decreased argininosuccinate synthetase (ASS) in liver with low amount of ASS protein and normal kinetic properties. Diurnal fluctuation of consciousness level in this patient was not correlated with the concentration of ammonia in serum, but with that in cerebrospinal fluid. Main metabolic pathway of ammonia in brain is considered to be localized in astrocytes which have receptors for glucocorticoids. Glucocorticoids induce additional glutamine synthetase activity. Circadian rhythm of cortisol level might play a role in diurnal fluctuation of consciousness level in patients with citrullinemia.