After 3 d of E2 administration, women will undergo a 24-h sampling study in the GCRC. Estradiol administration (E2 patches) will continue throughout the GCRC admission. Subjects will be admitted to the GCRC at 1800 h (2 h prior to sampling). Beginning at 2000 h, blood will be obtained through an indwelling i.v. forearm heparin lock over a 24-h period as follows: LH every 10 min (1 ml); P every 30 min (1 ml); FSH, E2, and T every 2 h (assays to be run in same samples as LH and P). SHBG, fasting insulin, and fasting glucose (i.e., fasting since 2200 h) will be run on the 0600 h sample (extra 2 cc drawn). (Subjects will fast from 2200 to 0600 h.) After 10 h of sampling (i.e., at 0600 h), either oral micronized P (100 mg p.o.) suspension or placebo suspension will be administered (according to randomization). With exogenous P, we aim to achieve mean plasma P concentrations 4-8 ng/ml. Subjects will not be allowed to sleep during the day (i.e., from 0600 to 2200 h). Subjects will be encouraged to sleep from 2200 to 0600. Sleep will be formally evaluated (extraoculograms, electroencephalograms, wrist actigraphy, etc.). At the completion of sampling, E2 patches will be discontinued. Volunteers will be discharged on oral iron (325 mg BID). We will ask women to eat only the food provided by the GCRC.