During childhood, the levels of certain hormones: gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), estrogen, and progesterone are very low. However, when puberty starts, GnRH and LH pulses begin to increase, but they initially do so at night only. It is unknown why GnRH and LH pulses increase at night and then decrease during the day (instead of being increased all the time). The purpose of this study is to see how quickly progesterone reduces LH pulses. The study is also meant to find out whether too much testosterone (also a hormone) in the blood causes problems with the ability of progesterone to reduce LH pulses. In this study, the investigators aim to discover whether or not giving 3 small doses of progesterone to pubertal girls will prevent the nighttime increase of LH pulses. From the information gathered in this study, the investigators may be able to learn more about how menstrual cycles are normally established in girls during puberty. Ultimately, if the investigators understand these normal processes, the investigators may be able to better understand abnormalities of puberty.