Changes in Peripheral Plasma Progesterone During the Rat 4-Day Estrous Cycle: An Adrenal Diurnal Rhythm 1

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.