d-Methylphenidate (d-MPH) was approved as a treatment for attention deficit hyperactivity disorder (ADHD) in children. The repeated-dose toxicity of the d enantiomer of d,l-methylphenidate (d,l-MPH) was assessed in male and female Beagle dogs. Dogs were orally dosed twice a day in equally divided doses 6 hours apart for total daily doses of 1, 3, and 10 mg/kg/day d-MPH or 20 mg/kg/day d,l-MPH for 90 days, followed by a 30-day recovery period. The top d-MPH dose of 10 mg/kg was equimolar to 20 mg/kg d,l-MPH in d-MPH content. The 10-mg/kg d-MPH and d,l-MPH doses were at least 13 times the maximum therapeutic dose giving rise to systemic exposures that were equivalent to or at least 2 times greater than those at the maximum therapeutic doses in children. The 10-mg/kg d-MPH and 20-mg/kg d,l-MPH doses had systemic exposures that were equivalent to or two to five times greater than the maximum therapeutic plasma levels in children respectively. There was no treatment-related mortality in all doses tested. Reversible salivation, hyperactivity, and diarrhea were seen in the high-dose d-MPH and d,l-MPH groups. Significant body weight loss and reduction in food consumption were observed in males for both high-dose groups with weights comparable to control values by the end of the recovery period. There were no abnormal clinical pathology or macroscopic or microscopic findings. Based on body weight changes, the no-observed-adverse-effect level (NOAEL) of d-MPH in beagle dogs was 3 mg/kg/day.