The pattern of histamine release has been investigated in various brain areas of anaesthetized cats and conscious, freely moving rats by the push-pull technique. In the hypothalamus, medial amygdaloid nucleus and mamillary body of the anaesthetized cat, histamine was found to be released according to an ultradian rhythm with a frequency of 1 cycle per 1-2 h. Additionally, oscillations have been observed in the medial amygdaloid nucleus and mamillary body with a frequency of 1 oscillation per 10 min. In the posterior hypothalamus of the conscious rat, histamine is also released rhythmically with a frequency of 1 cycle per 1.5 h. Moreover, the release rate of histamine is increased in the night.