We evaluated acutely salivary levels of cortisol before and after methylphenidate in children with attention-deficit hyperactivity disorder, and examined the change of cortisol after mental challenge task during MPH administration. 12 prepubertal boys with attention-deficit hyperactivity disorder-combined type were included into this study and diagnosed according to DSM-IV criteria. Three saliva samples from each subject were collected in days within and without methylphenidate at 08:00, 09:00, and 10:00 AM. In the period without methylphenidate treatment, the mean levels of salivary cortisol significantly decreased gradually in the course of time. In the period with methylphenidate treatment, there were no significant changes over time in the mean salivary cortisol levels. In conclusion, we suggest that methylphenidate significantly increases acute responses of salivary cortisol levels in children with attention-deficit hyperactivity disorder.