The goal of this study is to evaluate the use of clinical behavioral pharmacology methods, functional behavioral assessments (FBA), in assessing the treatment effects of pediatric medications in children with ASD. The present study of FBA procedures in pharmacological treatment will be conducted as a separate, but parallel study within IRB approved, federally funded, double-masked, placebo controlled medication trials of citalopram (GCO # 01-1295 PS*), an SSRI hypothesized to reduce stereotyped and repetitive behaviors in ASD and divalproex sodium (GCO # 01-0294), a medication recently found to reduce repetitive behaviors in ASD (Hollander et al., in press). This study will focus on the use of FBAs in distinguishing responders vs. nonresponders on the basis of behavior function, in evaluating functional patterns for stereotypy, aggression, and impulsivity, and in using descriptive FBAs as outcome measures in clinical trials.