Abstract
Summary
A diurnal rhythm exists in peripheral plasma progesterone concentrations during the 4-day estrous cycle of rats. Peak values in this rhythm were obtained during the early morning hours (0100-0500), whereas in the afternoon plasma concentrations of the steroid were at a nadir. Ovarian progesterone secretion rates within this same period remained unaltered, suggesting that the adrenal glands were responsible for these fluctuations.
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