Recently a number of investigators have suggested that body weight may be controlled by a neural system which contains a reference or set point. The finding that the body weight level which animals will defend can be altered by lesions of the lateral and ventromedial hypothalamus has been taken to provide support for this claim. In this paper we describe a simple feedback control model which contains no set point, and yet is able to account in full for these and other data which have been cited in support of the existence of a body weight set point. Since such a model exists it appears to be unnecessary and unparsimonious to introduce the concept of a neural set point to explain the fact that body weight maintains a relatively constant value which is defended against certain types of challenges. Constancy and defense of body weight are at best only prima facie evidence for the existence of a neural set point controlling body weight.