Considerable interest has been recently expressed in the theory that there is a set point which is responsible for regulating body weight. Supporters of this theory propose that anorectics reduce such a set point, and that food consumption is then secondarily affected to achieve a new body weight. We report in this study data obtained in rats which favor the set point theory. The major support for this conclusion is derived from an experiment in which the 'appetite suppressor,' fenfluramine, was administered in a paradigm which dissociated its ability to reduce food intake from its capacity to decrease body weight. Under these conditions, the body weight of a control group increased by 17 g; whereas that of the fenfluramine group was reduced by 11 g. In general, the fenfluramine-treated subjects ate as much as the controls. However, the controls increased body weight; whereas, the treated subjects demonstrated a decrease rather than an increase in this parameter. This result suggests that the fenfluramine group did not gain weight because the drug-altered 'set point' was lower in this group than in the controls.