This will be a randomized, double-blind, placebo-controlled, dose-ranging, efficacy and safety study in children with impulsive aggression comorbid with Attention-Deficit/Hyperactivity Disorder (ADHD). The target subjects are healthy male and female children aged 6 to 12 years, inclusive, with a diagnosis of ADHD. A total of 120 subjects will be randomized across approximately 30 US centers to one of four treatment groups.

Inclusion Criteria:

1. Healthy pediatric male or female subjects, age 6 to 12 years.
2. Diagnostic and Statistical Manual of Mental Disorders - IV -Text Revision (DSM-IV-TR) diagnosis of ADHD.
3. R-MOAS score >=24 at screening and R-MOAS score >=20 for randomization
4. IQ greater than 71.
5. Weight of >=20kg
6. current treatment with psychostimulant (1 month prior to screening)