The effects of two doses of d-amphetamine and fenfluramine on male Zucker rats maintained ad lib on solid and liquid diets were investigated using the technique of meal pattern analysis. Amphetamine-induced anorexia was of short duration in both obese and lean rats. In the lean rats, anorexia was followed by rebound feeding resulting in little or no reduction in total daily intake. The drug reduced meal sizes of obese but not lean rats and caused a transient decrease in meal frequency. Increased spontaneous activity paralleled the decreased food intake. In contrast, anorexia following fenfluramine was greater, more prolonged and of equivalent magnitude in obese and in lean rats. No rebound feeding was observed. Reduction in intake was achieved primarily by changes in meal size rather than in meal frequency. These data demonstrate that food intakes of genetically obese Zucker rats are more susceptible to the action of d-amphetamine than those of lean rats, and are consistent with reports of differential neurotransmitter levels in the obese and lean rats.