INCLUSION CRITERIA:  In order to be eligible to participate in this study, an individual must meet all of the following criteria:  Provision of signed and dated informed consent form Stated willingness to comply with all study procedures and availability for the duration of the study Having natural, spontaneous menstrual cycles (no hormonal therapy) less than 50 days in length. Aged 19-40 years Ability to take a vitamin D capsule and willing to adhere to the weekly regimen If sexually active, use of a non-hormonal contraceptive method.  EXCLUSION CRITERIA:  An individual who meets any of the following criteria will be excluded from participation in this study:  No menstrual period in the last 50 days, or, a typical menstrual cycle length <20 or >50 days. Use of a vitamin D supplement for more than one month. Volunteers will be asked to check their supplement label for vitamin D content. Unwilling to abstain from taking non-study vitamin D supplements during the study or if already taking a medically-advised vitamin D supplement. Use of a calcium supplement (including certain antacids) and unwilling to abstain from taking a calcium supplement during the study. Known polycystic ovarian syndrome Depo-provera use in the previous 12 months Current use of any hormones, including birth control Current use of an intrauterine device (IUD) Current pregnancy or lactation, trying to become pregnant, or planning to try in the next four menstrual cycles or 5 months. History of seizure disorders Celiac disease Crohn s Disease Body mass index >35 Aged <19 or > 40 years Presence of known contraindications for vitamin D supplementation, history of any of the following: calcium disorder or hypercalcemia, tuberculosis or granulomatous disease, metastatic bone disease, sarcoidosis, Williams syndrome, kidney disease (kidney stones, renal failure or dialysis, lupus nephritis) Known liver disease (cirrhosis) History of cancer other than skin cancer. History of anorexia nervosa, bulimia or an eating disorder Use within the past 60 days, of exogenous hormones Type 1 or Type 2 diabetes Known heart disease Gastric bypass surgery Thyroid disease such as hypothyroidism, hyperthyroidism, or Graves disease Unwilling or unable to complete study activities, e.g. collect daily urine specimens, have blood drawn, complete daily diaries, attend in person study visits