This is a supplement to project 5U01NS061264-05. This study is part of an Autism Centers of Excellence (ACE) Network. Previous research has shown that synthesis of the neurotransmitter serotonin is abnormal during critical periods of brain and synapse development in autistic children. The researchers hypothesize that one pharmacological approach to the treatment of autism is the use of serotonin agonists in children less than the age of six years. Following up on smaller pilot studies, this study is a randomized clinical trial of the serotonin agonist buspirone in a large group of young children with ASD. In this trial, researchers will test the safety and efficacy of buspirone in the treatment of core features of autism and associated behavioral dysfunction in young children with autism. Researchers will also employ a biomarker measure for serotonin synthesis capacity to assess patient differences in response to treatment. The researchers hypothesize that buspirone treatment effects on serotonin synthesis capacity will correlate with positive changes in social interaction, repetitive behavior, sensory dysfunction and anxiety. If buspirone is found to be safe and effective, it would represent a novel pharmacological treatment for core symptoms in ASDs.