The US has the highest unplanned pregnancy rate of any developed country (49%). Even women using birth control can experience an unplanned pregnancy. The number of US women taking oral contraceptives (OC) who experience an unintended pregnancy amounts to half a million over a year’s time. Many of these birth control failures occur because of women stopping their OCs due to perceived side effects, such as weight gain. Perceived weight gain was found to be the leading reason cited for discontinuation of oral contraceptives (OC) in US women. However, studies have not conclusively established if OCs do indeed cause weight gain. Interestingly, animal studies have consistently demonstrated a weight loss in menopausal females taking hormones similar to OCs. We believe that OCs decrease body weight but due to lifestyle and metabolism issues (eating habits, exercise, and decreasing metabolism with age), women gain weight over time and this is attributed incorrectly to OCs. Human studies to confirm the effect of OCs on body weight regulation have not been performed because controlling for food intake and activity level is difficult in humans. Nonhuman primates (monkeys) are ideal subjects to study the effect of OCs on weight because their reproductive system is similar to humans, but food intake and activity levels can be closely controlled and monitored. We propose to use this grant to test the hypothesis that OCs cause a decrease in body weight in primate species.