The investigators have previously shown a different time course in the developmental values of brain serotonin synthesis in children with autism using positron emission tomography (PET). These data are consistent with the notion that serotonin synthesis is abnormal during critical periods of brain development in autistic children. Specific to this project, the investigators demonstrated that serotonin synthesis capacity in children less than the age of six years showed significantly lower values than non-autistic children. Since serotonin is known to be an important factor involved in postnatal synaptogenesis, they hypothesized that one approach to the treatment of autism pharmacologically is the use of serotonin agonists in children less than the age of six years. The goal of this treatment is to provide a more normal serotonergic modulation of synaptic plasticity in autistic children during the early childhood years.