&NA;A comparison of the effects of lithium, haloperidol, and placebo on cognition is reported for a sample of hospitalized school‐age children with a behavioral profile of aggressiveness and explosiveness. In this double‐blind study, patients were randomly assigned to one of three treatment conditions. The cognitive battery was administered at the end of a 2‐week placebo baseline period and again after 4 weeks of treatment. It included a simple reaction time (RT) task with preparatory intervals of 1, 4, and 8 seconds, the Porteus Mazes, and the Matching Familiar Figures Test.Drug effects on cognition, when found, were mild. Slower and more variable RTs were found on the RT task in the haloperidol group (mean dose, 3.1 mg/day), particularly at the 4‐ and 8‐second preparatory intervals in comparison to placebo. This appeared to reflect decreased ability to hold a preparatory set. No other effects of haloperidol on cognitive performance were found. Lithium carbonate (mean dose, 1150 mg/day) caused a deterioration in qualitative performance on the Porteus Maze Test when compared with haloperidol but had no effect on test quotient scores or on the other cognitive measures. Results are discussed in terms of dose effects and the influences of task demands. This is part of a study critically assessing the effects of lithium and haloperidol on behavioral symptoms and other parameters.