**Study Overview:** This study aims to understand how the hormone *progesterone* can affect *luteinizing hormone* (LH) levels in girls during puberty. Puberty is when a child's body starts to change into an adult body. During this time, hormones like LH and *gonadotropin-releasing hormone* (GnRH) increase, especially at night. The study wants to know if taking progesterone can stop LH from rising at night and if the hormone *testosterone* affects this process. Participants will undergo tests to measure body fat, blood tests to check various hormone levels, and bone age x-rays. The study includes an overnight hospital stay, where blood samples are taken every 10 minutes over 20 hours while participants receive progesterone or a placebo. **Key Points:** - **Duration:** Includes a hospital stay lasting 20 hours for blood sampling. - **Procedures:** Blood tests, body measurements, and bone x-rays. - **Eligibility:** Girls in early to mid-puberty, not pregnant, and meeting specific health criteria. By understanding how these hormones work, scientists hope to learn more about normal and abnormal puberty.