In hemodialysates of cerebral venous blood from rabbits kept asleep by electrical stimulation of the somnogenic intralaminar thalamus, a seasonal correlation was found between the histamine (H) concentration and the external daily temperature, with a minimum in winter and a maximum in summer. By contrast, in dialysates of rabbits aroused by electrical stimulation of the midtrain reticular formation, the H concentration reached in winter the same high level as in summer. During the winter, stimulation of the waking midbrain reticular system increased the H-level of the cerebral blood dialysate, whereas that of the somnogenic intralaminar thalamus did not enhance, sometimes even lowered, the H-concentration. This suggests that the mechanism of H release in blood might be associated with the midbrain activating reticular system and moderated by the intralaminar thalamus.