The anorexic effects of three doses of amphetamine or fenfluramine were compared in rat pups 5, 10, 15, 20 and 25 days of age. The anorexic effects of the drugs were monitored by weighing the pups before and after a 90 minute feeding period following four hours of deprivation. Amphetamine appeared to induce weight gain in 5-day-old rats, but at 10 days and thereafter, became progressively more potent in reducing weight gain and caused weight loss indicative of the involvement of non-specific factors. In contrast, while fenfluramine also reduced weight gain at 10 days, its potency was reduced in older pups and it did not cause weight loss. These results suggest that in rat pups amphetamine and fenfluramine act differently to reduce feeding and that brain serotonergic systems mediating inhibition of suckling are functional in 10-day-old rats. It is also argued that these findings lend credence to the notion that the ontogeny of feeding parallels its recovery after brain lesions.