5-Hydroxytryptophan (5-HTP), amphetamine and fenfluramine suppressed food intake in normal rats and in aminals with lesions of the lateral hypothalamus. The anorexic effect of amphetamine was reduced in lesioned animals compared with controls while the effect of 5-HTP like that of fenfluramine was increased. When administered in conjunction with anorexic drugs, 5-HTP markedly potentiated the anorexic effect of amphetamine in both control and lesioned animals. However, 5-HTP potentiated fenfluramine anorexia only in lesioned rats. These findings provide further evidence for the role of 5-hydroxytryptamine (5-HT,in the anorexic effect of fenfluramine, and suggest that a 5-HT mechanism, inhibitory for feeding, produces particulary severe suppression of food intake in rats with lateral hypothalamic lesions.