1. 1. Plasma cortisol concentration (PCC) was determined in euthermic woodchucks (M. monax) for 3 consecutive days at hourly intervals. Cortisol was secreted in a highly episodic manner throughout the 24-hr day. Within 2 hr of lights on PCC rose sharply, but transiently. PCC was significantly higher during the lights on portion of the experiments. At lights off, cortisol levels fell and remained low until a few hours before lights on. 2. 2. The major peak of cortisol release was in the early morning, usually between 0600 and 0900 hr. A second smaller peak was found just before lights off (2000 hr). The cortisol secretion pattern strongly suggests a circadian rhythm of cortisol is present in euthermic woodchucks. 3. 3. There was no significant difference in cortisol concentration between fall and spring animals, and there was no difference between ages. Thus, cortisol secretion continues during the fall when the adrenal cortex is presumed to be involuted in preparation for hibernation. © 1980.
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