SUMMARY1. 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 ofthese circadian rhythms in relation to states of sleep and wakefulness, general metabolism, and motor activity is discussed.

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1. The circadian patterns for onset and duration of sleep after the administration of a constant dose of pentobarbitone sodium have been measured in rats adapted to and maintained under a fixed illumination cycle. The relationship of these patterns to others for rectal temperature and motor activity as well as for noradrenaline (NA), 5‐hydroxytryptamine (5‐HT) and histamine in mid‐brain and caudate nucleus, and histamine and glucose in blood serum, has been examined. 2. The onset of sleep is longest and duration shortest during the phase of the illumination cycle when motor activity and rectal temperature are maximal. An inverse relationship for these parameters is found during the light phase of the illumination cycle. 3. In untreated rats, mid‐brain NA and histamine and caudate nucleus histamine levels are maximal and 5‐HT minimal during the dark phase of the illumination cycle. An inverse relationship for these parameters is found during the light phase of the illumination cycle. 4. Pentobarbitone sodium treatment significantly elevates mid‐brain NA and histamine and caudate nucleus histamine levels during the dark phase of the illumination cycle. Although 5‐HT levels are reduced over the entire circadian cycle this change is significant only during the light phase of the illumination cycle. 5. Pentobarbitone sodium treatment reverses the circadian pattern for body temperature, producing a mirror image of the control pattern. 6. The circadian pattern for blood serum histamine levels differs from the C.N.S. pattern for histamine. Peak levels occur at the end of the light phase of the illumination cycle. After pentobarbitone sodium these levels are reduced, although the circadian pattern is similar to the control. 7. Circadian blood glucose patterns have bimodal peaks; a primary peak at the end of the dark phase, a secondary peak at the end of the light phase of the illumination cycle. Pentobarbitone sodium did not significantly alter the pattern. The relationship between this pattern and C.N.S. and peripheral histamine and catechol amines is discussed.

The administration of tremogenidine doses of Tremorine, 1,4-dipyrrolidino-2 butyne, is followed by a significant decrease in the concentration of norepinephrine in the brain stem of three common laboratory species. The change in the concentration proceeds at a rate which coincides with the occurrence of the tremor in each of these species. In the rat, the change in norepinephrine is followed by a progressive increase in the concentration of 5-hydroxytryptamine in the brain stem. Bilateral adrenalectomy in the rat enhances the Tremorine-induced changes in the concentration of norepinephrine and antagonizes the increase in the concentration of S-hydroxytryptamine.

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