Rats maintaining reduced body weights after lesions of the lateral hypothalamus (LH; LH rats) are characterized by smaller body protein masses. Two experiments were conducted to determine whether this reduced protein mass is actively defended. In the first, it was found that LH rats induced to overeat and restore body weight to the level of nonlesioned controls markedly increased their body fat without significantly increasing body protein. That is, LH rats at normal body weights were notably obese. In the second experiment, body protein losses produced by food restriction in LH rats were both relatively small and proportionally the same as those seen in similarly restricted nonlesioned controls. These observations demonstrate that LH rats retain the capacity for preserving body protein when challenged by either under- or overnutrition. The apparently irreversible reduction in the body protein mass thus appears to be the result of a specific lean tissue downregulation induced by LH damage.