Activity of the pituitary-adrenocortical system and thyroid gland during the oestrous cycle of the rat.

The concentrations of LH, progesterone, ACTH, corticosterone and thyroxine in the plasma were estimated at various times during the oestrous cycle of the rat. The well-established patterns of LH and progesterone secretion were confirmed. On each day of the cycle the plasma concentrations of ACTH and corticosterone were lowest in the morning and rose in the afternoon. Conversely, during oestrus and dioestrus, the plasma concentrations of thyroxine were higher in the morning than in the evening. However, during the afternoon of pro-oestrus the concentrations of ACTH, corticosterone and thyroxine in the plasma rose and, like the concentrations of LH and progesterone, all reached levels far higher than those attained at any other time of the cycle.