The effect of aspartame on the learning, behavior, and mood of children was evaluated in two experiments. After an overnight fast and a standard breakfast, 20 healthy 9- to 10-year-old children were given the treatments in a double-blind crossover design at 10:30 AM. Lunch was served at 12:00 noon. In experiment 1, the treatment consisted of an ice slurry of strawberry Kool-Aid containing 1.75 g/kg of carbohydrate (polycose) plus either aspartame (34 mg/kg) or the equivalent sweetness as sodium cyclamate and amino acids as alanine. In experiment 2, the treatment consisted of a drink of cold unsweetened strawberry Kool-Aid, containing either 1.75 g/kg of sucrose or 9.7 mg/kg of aspartame. Measures of associative learning, arithmetic calculation, activity level, social interaction, and mood were unaffected by treatment in experiment 1. In experiment 2, the only significant treatment effect was that on the frequency of minor and gross motor behaviors, which were less frequent after the consumption of sucrose than after aspartame. Thus, the effect of aspartame on the short-term behavior of healthy 9- to 10-year-old children appears to be related to its absence of metabolic consequences rather than to its amino acid composition and putative neurochemical impact.