An alimentary/thermic conflict of motivation was used to explore the effects of very low doses of dexfenfluramine (dFF), an anorectic serotoninergic agonist, on the parameters of food motivation, drive and incentive (or palatability). Six rats trained to feed 2 hr/day, were given the possibility to feed on chow in a shelter (25 degrees C), and to get a snack of shortcake, a highly palatable bait, from a feeder placed 16 m away in a very cold environment (-15 degrees C). dFF at 0.6 or 1.25 mg/kg decreased neither the chow intake in the shelter, nor the mean duration of the snacks in the cold, which is the parameter believed to be the best indicator of incentive. In contrast, dFF reduced the number of trips to the bait in the cold as well as the total mass of palatable bait ingested and the mean amount ingested by snack. Such an effect was no longer observed after a food restriction had reduced the body weight of the rats to 90% of its initial value. It is concluded that, even at doses too small to reduce the consumption of basic food, dFF decreases the drive to get palatable food.