Estrogen levels in rat ovarian venous plasma were determined by an intravaginal bioassay. During the estrous cycle, estrogen secretion rate rose in the afternoon of diestrus (the day prior to proestrus) and continued to rise gradually throughout the night of diestrus. Maximal secretion rate was observed between 0500 and 2000 hr of proestrus: 9–11 ng estradiol equivalent (E.E.)/ovary/hr. A major fall in secretion rate was observed between 2000 and 2300 hr of proestrus. Slight rises in secretion rate were observed in the afternoons of estrus and metestrus. The changes in estrogen concentration in ovarian venous plasma were similar but the major peak on the day of proestrus was sharper, being at about 1000 to 1500 hr on account of changes in blood flow. During pregnancy, estrogen secretion rate was generally low except near term. A significant increase in estrogen secretion rate was observed in the afternoon of Day 4 (2.3 ng E.E./ovary/hr); secretion had fallen by the night of the same day. Near term, a rapi...
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Yoshinaga et al. (1969) studied this question.