Research geared towards increasing knowledge of the variables contributing to treatment response is critical to advancing intervention studies in ASD. Literature in the growing field of clinical behavioral pharmacology suggests functional behavioral assessments (FBA), when integrated into clinical trials of medication, can be important tools in elucidating the specificity of treatment effects (Mace & Mauk, 1995, Northup & Gully, 2001). The goal of this study is to evaluate the use of clinical behavioral pharmacology methods, primarily descriptive functional behavioral assessments (FBA), in a double-masked, placebo controlled trial of medications used to treat children with autism. These medications are hypothesized to reduce the stereotyped and repetitive behaviors in ASD. This study will focus on the use of descriptive FBAs in distinguishing responders vs. non-responders on the basis of behavior function, in evaluating functional patterns for stereotypy, and in using descriptive FBAs as outcome measures in clinical trials.