In this study, dietary intervention before and after weaning was used to produce body fatness in adult rats ranging from severe obesity (61% body fat) to the lean condition (18% body fat). To produce severe obesity, rats were programmed for a high caloric intake during suckling. After weaning, they were provided unrestricted access to a diet in which approximately 40% of the calories were derived from fat (high-fat diet). The high-fat diet appeared to cause hyperphagia such that appetites were highly stimulated between 29 and 58 wk of age. Body weight in these rats stabilized at 1,213 +/- 62 g. Severe obesity was also observed in rats programmed for a low caloric intake if, after weaning, they ate the high-fat diet (59% body fat). These results provide evidence that the fat content of the diet may play a regulatory role in the development of severe obesity.