Research has implicated the deep cerebellar nuclei in autism. This study questioned whether fastigial nuclei abnormalities account for some of the irregular social behaviors seen in autism. Bilateral cannulation surgery was performed on 13 rats. An ABABAB reversal design was implemented. All animals received a microinfusion of saline during the baseline (A) phases. The experimental animal was placed in an open field with an unfamiliar confederate animal, and social interactions between the two animals were measured for 10 min. Seven animals received microinfusions of bupivacaine during the treatment phases (B), which temporarily inactivated the fastigial nuclei. Six control animals received saline again, and social interaction was retested. This sequence was executed 3 times over 6 days to achieve an ABABAB reversal design. Because the cerebellum is involved in motor behavior, the animals' locomotion was analyzed to ensure results were not because of locomotor deficits. A gait analysis and distance traveled in the open field were used to measure locomotion. No differences were found in locomotor behavior. Results of the social interaction analyses indicate animals with inactivated fastigial nuclei engage in less intense social interactions and engage in more behaviors to prevent social interaction. Knowledge that the fastigial nuclei mediate social interaction can further the understanding of pathology in the autistic brain and lead to breakthrough treatments. (PsycINFO Database Record (c) 2018 APA, all rights reserved).