The circadian rhythms in both food intake and plasma corticosterone (11-OHCS) in adult female rats were altered concomitantly under various lighting conditions. In all lighting regimens, plasma 11-OHCS levels tended to be elevated when rats were inactive and mainly asleep and decreased when they were active and ate vigorously. When illumination was changed from a normal to an inverted light-dark cycle, the diurnal variations in food intake and plasma 11-OHCS coincidentally shifted and inverted on the 3rd day and thereafter. Thus, it is demonstrated that the circadian rhythm of the pituitary-adrenal system is clearly associated with the daily rhythm of motor activity represented by eating behavior, regardless of lighting conditions. After rats adapted to restricted feedings in various schedules, they demonstrated altered circadian rhythms of plasma 11-OHCS even under the normal light-dark cycle. The highest values of plasma 11-OHCS generally occurred before rats began to eat, although these values were not always observed just prior to feeding. After the rats had adapted to restricted feeding during the day-time, up to the day before sacrifice (they were allowed to feed ad libitum on the day of sacrifice), they still demonstrated an altered circadian 11-OHCS rhythm with a peak value at noon. These results suggest that the periodicities in sleep-waking, rest-activity or fasting-eating may be more potent and important a synchronizer of the pituitary-adrenal system’s circadian rhythm than the light-dark cycle, and that the elevation of plasma 11-OHCS before feeding may not be caused by the restriction of feeding per se but from the alteration of rest-activity cycles induced by the change of the feeding behavior.

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