PROJECT SUMMARY/ABSTRACT In adolescent girls, loss of control (LOC) eating is a maladaptive eating behavior associated with psychiatric morbidity, including the development of full syndrome eating disorders. However, the daily biobehavioral factors that precipitate LOC eating are not well known. Ovarian hormone levels are key biological factors associated with the etiology of eating disorders in adolescent girls. Yet, models on how daily ovarian hormone exposure predicts LOC eating in adolescent girls are underdeveloped. Two relevant research domain criteria (RDoC) implicated in eating disorders include reward anticipation and response inhibition. These processes associate with variations in ovarian hormone levels, and thus, daily hormone exposure may cause dysregulation in reward anticipation and response inhibition, which may in turn precipitate LOC eating. Given the substantial gap in research on daily hormone exposure and LOC eating in adolescent girls including momentary mediating mechanisms, the goal of this study is to examine the patterns and mechanisms of ovarian hormone exposure on LOC eating across the menstrual cycle in adolescent girls. Ecological momentary assessment (EMA) paired with daily hormonal sampling will be used to examine daily and momentary associations between daily hormone exposure and LOC eating and the mediating role of momentary fluctuations in reward anticipation and response inhibition. Normally cycling adolescent girls who have reached menarche will provide daily saliva samples for hormone analysis and be administered EMA for 35 days on mobile phones. During EMA, girls will report LOC eating and will complete measures of response inhibition and reward anticipation. This study has two specific aims. Specific Aim 1 is to characterize the association between daily hormone exposure and LOC eating in adolescent girls, and Specific Aim 2 is to examine a mediation model of daily hormone exposure and momentary RDoC constructs (i.e., response inhibition and reward anticipation) in predicting LOC eating. This work has implications for the development of new real-world biobehavioral models of LOC eating in adolescent girls within the RDoC framework, which will guide future theoretical developments and treatment for LOC eating. For example, results will provide preliminary evidence for treatment targets for novel interventions for adolescent girls—e.g., a response inhibition intervention may be needed during phases of the menstrual cycle characterized by specific hormonal profiles.