Abstract
1. The circadian rhythms of rectal temperature and biogenic amine levels in mid-brain and caudate nucleus have been measured in normal and adrenalectomized rats adapted to and maintained under fixed illumination cycle.2. Rectal temperature reaches a peak value between 24.00 hr and 06.00 hr during the dark phase of the illumination cycle at a time when motor activity is maximal. In adrenalectomized rats, the pattern is similar but the peak is significantly lower.3. Highest histamine levels in the caudate nucleus and mid-brain of normal and adrenalectomized rats are found at the time when body temperature and motor activity is maximal.4. Similarly, caudate nucleus and mid-brain noradrenaline levels reach their peak during the dark phase of the illumination cycle. These levels are significantly different from those found during the light phase of the illumination cycle. The rise in noradrenaline in the mid-brain of adrenalectomized rats, however, was not significant.5. Peak 5-hydroxytryptamine (5-HT) levels in the caudate nucleus of normal rats and the mid-brain of adrenalectomized rats were found to be 12 hr out of phase with peak values obtained for other parameters that were measured.6. The significance of these circadian rhythms in relation to states of sleep and wakefulness, general metabolism, and motor activity is discussed.