Circadian rhythm of plasma growth hormone levels in the pigeon∗∗

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Abstract
The plasma growth hormone (GH) level in the pigeon was assessed by radioimmunoassay during a 24‐hr period of 12‐hr light (photophase) and 12‐hr dark (scotophase). The peak GH level was found to be at 06.00 hr (toward the end of the scotophase or onset of the photophase). Thereafter it dropped to its lowest at 18.00 hr (end of the photophase), thus revealing the existence of a circadian rhythm. This rhythm in plasma GH concentration has been correlated with a corresponding free fatty acid rhythm in the pigeon. Diurnal changes, free fatty acid levels, lipid mobilization, muscular activity, plasma growth hormone, starvation.