Riluzole, a glutamate antagonist, reduces compulsions (including repetitive behavior) in adults with obsessive-compulsive disorder (OCD). Given the close similarity between symptoms in childhood-onset OCD and the frequency with which OCD is present as a co-occuring condition in autism spectrum disorder (ASD), it is postulated that medications which reduce OCD symptoms might also improve the repetitive behaviors and fixated interests of ASD. Preliminary data will be collected on riluzole efficacy and safety in children and adolescents with co-occurring conditions of ASD and OCD. Additionally, a study of the neurochemical effects of riluzole treatment is underway in which children and adolescents will undergo a magnetic resonance spectroscopy (MRS) scan (similar to a magnetic resonance imaging (MRI) scan) before starting treatment with riluzole and during treatment. Changes in brain concentrations of glutamate and related chemicals will be assessed and compared with symptom severity and degree of treatment response.