The preovulatory follicles of the domestic hen are arranged in a hierarchy. The largest follicle (Fl) secretes predominantly progesterone whereas the second largest follicle (F2) and the third largest follicle (F3) produce primarily testosterone and estradiol, respectively. The granulosa cells secrete progesterone and testosterone and theca cells secrete testosterone and estradiol. The cause(s) for the termination of estradiol synthesis is (are) not known. Therefore, the focus of this proposal is an investigation of the mechanism(s) which cause this inhibition of estradiol synthesis by the F3 follicle. We will measure testosterone and estradiol secretion by the F2 and F3 theca cells during the cycle by incubating isolated theca cells + the appropriate substrate. Second, we will examine the effects of repeated or sequential challenges of LH, FSH, or LH:FSH on steroid secretion by isolated granulosa and theca cells layered on Bio-Gel beads in perifusion chambers. Third, we will investigate the role of progesterone as the possible mediator of the gonadotropin induced inhibition of estradiol synthesis by the theca cells. In summary, we hope to make the following conclusions from the proposed work: 1) there is a loss of aromatase enzyme activity as the follicles mature, suggesting that this enzyme is sensitive to inhibition; 2) the termination of estradiol biosynthesis by the theca is the result of repeated exposures of the maturing follicle to LH; 3) the action of LH is mediated through increased secretion of progesterone by the granulosa cells; 4) the function of theca cells, specifically steroid biosynthesis, is modulated by the granulosa cells; and 5) progesterone inhibits estradiol synthesis through a non-competitive mechanism.