This study investigated whether the stimulant methylphenidate (Ritalin) would differentially affect performance on a test of cognitive style by hyperactive children. Subjects were 55 children, (ages 7 to 15) referred to the Learning Clinic at the Hospital for Sick Children, Toronto, Canada for hyperactivity and/or learning problems. A paired associate learning task was used to classify subjects as favorable or adverse responders to stimulant medication. Favorable responders included two distinct clinical groups: those described by teachers and parents as aggressive, impulsive, and acting out, and those referred for daydreaming, distractibility, and problems in maintaining attention. All subjects were given Kagan's Matching Familiar Figures (MFF) test, a test of reflective/impulsive cognitive style. It was found that stimulant medication (Ritalin) resulted in poorer MFF performance for adverse responders but improved performance for the impulsive and distractible groups. Results suggest that stimulants affect cognitive style differentially depending on the child's clinical/behavioral profile. (Author/SE)