Long-term infusion of glucose, beta-hydroxybutyrate, and glycerol into the third ventricle of the rat brain caused a stabilization of body weight at a lower than normal level. Among the glucose- and glycerol-treated animals this weight loss was caused in part by temporary hypophagia. Among the animals treated with beta-hydroxybutyrate the weight loss was unaccompanied by a reduction in food intake. The results are consistent with the view that the systems controlling food intake and body weight are sensitive to the availability of brain fuels. They are not consistent however, with the view that these control systems monitor calories independently of their source.