 The investigators specific aim is to examine the effects of serial doses of propranolol in combination with early intensive behavioral intervention on social interaction, and secondarily on language tasks, anxiety, adaptive behaviors, and global function in children with autism in a double-blinded, placebo-controlled trial. The investigators will also examine whether response to treatment can be predicted based upon markers of autonomic functioning, such as pupillary light reflex and heart rate variability, and whether anxiety predicts treatment response. The investigators hypothesis is that propranolol will augment the effects of behavioral therapy on social functioning and language abilities. The investigators have demonstrated benefits of propranolol on social functioning and language in previous single-dose studies in adults and adolescents with autism. The investigators also predict that those with the greatest degree of autonomic dysregulation will demonstrate the greatest benefit from the drug. 