Determination of Mechanisms of Hyperestrogenemia in Perimenopausal Women. This experiment will study a total of 90 women. Fifty will be perimenopausal and 40 will be midreproductive aged (strict definitions will be used, based upon the PI's previous data). Serial ultrasound studies will be performed to characterize ovarian morphology up to and immediately beyond ovulation. In a subset of women (12 midreproductive aged and 20 perimenopausal), daily serum sampling will be performed to assess the patterns of LH, FSH, estradiol, progesterone and inhibin A and B and activin A and B secretion, as these are the hypothesized peptides that regulate FSH. This study is approximately 25% completed at this point, and has been chiefly hampered by slow recruitment. As we now seem to have discovered an effective way to recruit women, we plan to complete this study within the coming year.