Attention deficit hyperactivity disorder (ADHD) is the major psychiatric illness affecting school-age children, characterized by high levels of hyperactivity, inattention and impulsivity. Most children diagnosed with ADHD will be treated with stimulants such as methylphenidate (MPH) or Adderall(amphetamine). Over the last 10 years, an exponential increase in stimulant use in Canada has raised concerns about the long term consequences of such treatments as they are extended to younger children (preschoolers)and for longer periods of time (years to decades). Since a child's brain continues to develop well into the second decade of life, the long term use of such drugs during this critical phase of brain development could have important consequences on brain functioning but surprisingly little is known about the developmental effects of these drugs. Animal models where we can administer MPH during brain development are an effective way to gain vital information on how these medications may alter the genes involved in important brain networks controlling attention, learning and possibly predisposition to addiction.We have already reported that chronic treatment of young, prepubertal rats with MPH downregulates gene expression in several brain areas.In addition we have developed a novel,non-stressful technique to simulate the oral treatment of children with psychostimulants, and we confirmed, by mass spectrometry, that blood levels of MPH in our young rats are the same as those in children.By establishing the effects of MPH on gene expression in the developing brain, we hope to create standards where all drugs will be tested for their effects on the brain before they are administered to young children.( Funded by CIHR/NSHRF/IWK; Scottish Rite Canada; Atlee Endowment).