This is an open-label Phase 1 study in healthy premenopausal females evaluating the effects of different dosing regimens of elagolix on ovarian activity, ovulation, and ovarian reserve and to assess the effect of elagolix on selected endocrine/hormone levels.

This is an open-label, multiple-dose, Phase 1 study in premenopausal subjects aged 18 years to 40 years, with a history of regular menstrual cycles (24 to 32 days long) and no evidence of significant gynecological disorders. The objective of the study is to determine the effects of different dosing regimens of elagolix on ovulation, ovarian activity, and ovarian reserve. The study consists of 3 periods: a Screening Period of up to 50 days prior to the first dose, a Treatment Period of 3 months duration (Cycles 1-3), and a Follow-up Period of up to 60 days. During the Screening and Treatment menstrual cycles, serial transvaginal ultrasounds and determination of serum levels of luteinizing hormone, follicle-stimulating hormone, estradiol, progesterone, and inhibin B will be performed three times weekly. Subjects will maintain a daily diary of uterine bleeding. Pregnancy testing will be performed frequently throughout the study. Subjects will be required to use nonhormonal dual contraception consistently during the study, and will be counseled on appropriate and effective forms of birth control to promote pregnancy prevention.

Inclusion Criteria:

- Premenopausal female, between 18 and 40 years of age, inclusive - History of regular menstrual cycles - Endocrine and ultrasonographic evidence of ovulation and normal ovulatory cycle during the screening period - Follicle-stimulating hormone level of <35 mIU/mL - Agrees to use required birth control methods during the entire length of participation in the study

Exclusion Criteria:

- Screening ultrasound results show a clinically significant gynecological disorder - Surgical history of hysterectomy without oophorectomy, unilateral or bilateral oophorectomy, removal of ovarian cysts - Less than 6 months postpartum or post-lactation at the start of study drug dosing - Pregnant or breast feeding or is planning a pregnancy within the next 12 months - Testosterone concentration >120 ng/dL at screening