PROJECT SUMMARY At this time, alcohol is the top substance used by US emerging adults under the legal drinking age (underage emerging adults; U-EA; ages 18-19). While drinking rates had recently been trending downward, 2021 national data reflect rises in youth drinking across all metrics. This is highly consequential in terms of youth safety and neurodevelopment. One of the central challenges of U-EA alcohol use is that youth are unlikely to seek, receive, or complete indicated alcohol intervention. In turn, it is imperative to find brief, effective interventions to impactfully intervene with U-EA hazardous alcohol use. Additionally, the developmental neuroscience literature robustly reflects that peers hold higher neural salience during this developmental window, as evidenced by youths’ differential neural response in conditions with real and/or simulated peers, even when those peers were not friends, and particularly in the context of alcohol. Furthermore, studies are recognizing that peers concomitantly activate positive (prosocial) neural and behavioral responses in this age group, and this has been observed in conditions of negative and positive peer feedback. The role of negative and positive peer feedback is consequential given that one of most widely-used U-EA intervention service delivery methods are group-based formats. Here, we aim to build upon PI Feldstein Ewing’s 15 year history of continuous NIH funding in youth translational (brain:behavioral) approaches evaluating youth within-session factors, youth neural response, and subsequent behavior change. Across independent samples, our team has found distinct developmental brain response to MI interventions, largely localized to default mode network (DMN) regions [precuneus, posterior central cortex (PCC)]. Further, this observed DMN activation to examined within-session factors (client language; therapist language) was significantly associated with youth post-treatment behavior change. We thus respond to PAR-21-280: “Dyadic Interpersonal Process and Biopsychosocial Outcomes” and propose functional magnetic resonance imaging (fMRI) to evaluate U-EA brain response in the underexamined, but widely-utilized, group MI context. study, we will examine the nature of peer-peer dyadic exchanges in the youth brain during group MI In this interventions (Aim 1) and predictors of behavior change: prospective relationships between youth brain response and behavior change in the novel context of group MI (Aim 2). These data are crucial for guiding improvements in brief behavioral group-based intervention programming alcohol use. for U-EA engaged in hazardous