Twenty-eight children (ages 5-13) with mental retardation, hyperactivity, and inattentiveness were administered fenfluramine and methylphenidate. Fenfluramine was superior to placebo on the memory task, whereas methylphenidate reduced commission errors on a continuous performance test. Methylphenidate caused shorter response times whereas fenfluramine caused increases. Improvements were observed in attention, activity level, and mood with both drugs. (Author/JDD)