We propose to study children with ADHD with single and paired pulse TMS to determine if the clinical abnormalities present in these children reflect an underlying delay or abnormality in maturation of the inhibitory and facilitatory systems of the cortex and corticospinal tracts. If present, these findings would give objective evidence of abnormal inhibitory systems in attention-deficit/hyperactivity disorder; their presence (or absence) may differentiate between the different subtypes of ADHD. Since stimulants have an ameliorative effect on the neuropsychological functioning in these children, future studies would be able to determine whether they have a similar effect on the motor system.