The current study is designed to determine: 1) whether negative feedback on LH and FSH occurs at the pituitary; and 2) whether there is an effect of aging on estrogen negative feedback at the pituitary. Younger and older postmenopausal women underwent a baseline study and a second identical study after a month of low dose estrogen replacement. The study protocol consisted of the following: 1) administration of a GnRH antagonist (Nal-Glu at 150 mg/kg that blocks endogenous GnRH so that the dose and interval of pituitary exposure to GnRH are precisely controlled; 2) beginning 8 hours following GnRH antagonist administration (at a time when LH had reached its nadir following GnRH receptor blockade), administration of 4 graded doses of GnRH (25, 75, 250 and 750 ng/kg every 2 hours with 2 hours of blood draws following each dose). Blood was sampled every 30 min for 4 hours before antagonist administration, every 30 min for the following 7 hours and then every 10 min until the completion of the study.