The present study investigated the effects of methylphenidate (MPH) on the resting heart rate of 47 children with Attention Deficit Disorder/Hyperactivity. Children participated in a double-blind, placebo control, within subject (cross-over) design and received each of 5 doses of MPH in a counterbalanced sequence. A multi-step analysis was used to examine the dose-response relationship, contribution of pre-stimulus levels of HR (i.e. the initial value), and potential interaction between dosage and time on resting HR. Results indicated that higher dosages are linearly related to increasing levels of HR and that these effects are dependent upon both the initial HR value and the time course of the medication.