PROJECT SUMMARY One in 54 children have Autism Spectrum Disorder (ASD) in the U.S., defined by impaired social interaction and restricted, repetitive behaviors. Impaired self-regulation is believed to be inherent in ASD, as evidenced by difficulty managing emotions, heightened physiological reactivity to daily activities, and increased problem behaviors such as hyperactivity, irritability, aggression, elopement, and self-injury. Growing evidence suggests that interacting with animals promotes self-regulation in youth with ASD. Integrating animals, such as horses, into therapy for youth with ASD may improve outcomes. However, very little is known about the efficacy of integrating horses into occupational therapy for youth with ASD, despite its growing prevalence in practice. This knowledge gap results in scant empirical information to guide occupational therapy and creates barriers to access services, as many third-party payers do not reimburse for occupational therapy that integrates horses. Consequently, there is a critical need to establish how to best integrate horses into occupational therapy for youth ASD, and empirically demonstrate the additive benefit horses have on therapy outcomes. Our long-term goal is to establish integrating horses into occupational therapy as an evidence-based practice to improve self-regulation in youth with ASD. Our team has created a theory-driven, standardized intervention manual called “Occupational Therapy in an Equine Environment: Harnessing Occupation for self-Regulation Skills” (OTEE HORS). OTEE HORS operationalizes the purposeful integration of horses into an occupational therapy intervention to optimize development of self-regulation skills. We also manualized a clinical treatment protocol, Occupational Therapy in a Clinic (OT Clinic), to serve as a control condition for all non-animal elements of the intervention. We have demonstrated that both OTEE HORS and OT Clinic are feasible to implement and acceptable to parents, youth, and occupational therapists. Our preliminary efficacy data suggest that both OTEE HORS may improve self-regulation in youth with ASD, but we have not yet compared OTEE HORS to OT Clinic. The goal of this project is to quantify the unique benefits of integrating horses in occupational therapy compared to occupational therapy in the clinic (i.e., OTEE HORS vs. OT Clinic) for improving self-regulation in youth with ASD. Sixty-four youth with ASD ages 6-11 years old will be randomized to OTEE HORS (N=32) or OT clinic (N=32). The specific aims are to 1) estimate the preliminary efficacy of OTEE HORS compared to OT Clinic on self- regulation, and 2) identify potential physiological mechanisms that explain how integration of horses in occupational therapy impacts self-regulation (salivary cortisol and salivary alpha-amylase). Our expected outcomes include preliminary efficacy data and identification of physiological mechanisms that explain how integrating horses into occupational therapy ...