One of the most effective treatments for children with Attention Deficit Disorder with Hyperactivity (ADDH) has been the prescription of methylphenidate (MPH). While laboratory-based evidence indicates that MPH effects may be rate-dependent, little is known about changes evinced in operant behavior controlled by complex reinforcement schedules. The present study examined the effects of several doses of MPH on the operant key-pressing behavior of 20 children with ADDH who were favorable responders to the drug. Following a drug-free training period, each child performed the task across all doses in a randomly assigned sequence under double-blind placebo control conditions. Rate-dependent effects were found for both high- and low-rate reinforcement schedules. The strength of these effects varied across dose. These results have implications for the nature of rate-dependent phenomena in humans and investigations of treatment parameters in the pharmacotherapy of children with ADDH.