To determine whether central insulin administration lowers the level around which body weight is regulated, insulin (6 mU/day) or saline was infused into the third ventricles of four groups of rats. One insulin-infused and one saline-infused group were food-deprived for 3 days and were then returned to an ad lib feeding schedule. The other two groups were maintained on ad lib feeding throughout. Insulin-fused food-deprived rats. In ad lib fed rats, insulin caused a significant reduction of food intake and weight relative to saline-infused controls. When formerly food-deprived rats were returned to ad lib feeding, they gained weight, and this was significantly more pronounced in the saline-infused than the insulin-fused group. The body weights of the two insulin-infused groups converged on a value approximately 9% below the average of the two saline infused groups, with one group increasing its weight and the other decreasing its weight to achieve that weight. These findings suggest that the third-ventricular infusion of insulin does not incapacitate the rats and that they can alter their food intake either upward or downward to attain a new weight. The results are also consistent with the hypothesis that direct administration of insulin into the brain determines the level of weight maintained by the animal.