Neurobiological findings have led the way to a consideration of certain therapeutic agents for autism and fragile X syndrome (FXS). Research into the neurobiology of autism initially focused on the monoamine neurotransmitter serotonin (5-hydroxytryptamine [5-HT]). FXS is the most common inherited form of mental retardation. The disorder is the result of the silencing of a single gene on the X chromosome known as the fragile X mental retardation 1 gene (FMR1). Attention-deficit/hyperactivity disorder (ADHD) is a heterogeneous condition characterized by interfering symptoms of hyperactivity, impulsivity, and inattention. Investigations into the role of dopamine in the pathophysiology of ADHD have involved peripheral and central measures of dopamine and its main metabolite, homovanillic acid (HVA). Although multiple paths may lead to the identification of a novel pharmacotherapy, with each new discovery comes the opportunity to meaningfully impact the lives of individuals suffering from these neurodevelopmental disorders.