This study was designed to investigate the effect of varying concentrations of estradiol, administered to normal women, upon the gonadotropin response to synthetic gonadotropin-releasing hormone (GnRH or LRF). Beginning at 4 pm on the first day of the menstrual cycle, 19 studies were performed in subjects who received injections of estradiol benzoate (E2B every 12 h for 6 days). Concentrations of E2B administered (mug/kg/12 h) were: 0.3, 0.6, 1.25, 2.5, 3.75, and 5.0. Mean serum estradiol concentrations achieved at these respective concentrations of E2B were 43, 53, 91, 145, 195, and 305 pg/ml. Twelve h after the last E2B injection, an intravenous bolus of 100 mug GnRH was administered. Gonadotropin response to this dose of GnRH after E2B was compared with each subject's response in the early follicular phase of a previous (control) cycle during which no exogenous estradiol was administered.