Graded increments in the fat-to-carbohydrate ratio of the diet elicited a gradual, but reversible increase in the average mass of body fat maintained by adult female (CDI) albino mice under ad libitum feeding conditions. In addition, the inter-individual variability in the animals' fat mass was greatly magnified by diets with a substantial fat content (greater than 30 percent of calories). As a result, the incidence of obesity (defined as one third or more of body weight as fat) increased progressively from 0 percent to 35 percent when the diet's fat content was varied from 1 percent to 64 percent of its total energy content. A state of weight maintenance can only become established when the relative rates of glucose and fatty acid oxidation are proportional, on average, to the carbohydrate-to-fat ratio of the diet. When diets with a relatively high fat content are consumed, a considerable enlargement of the adipose tissue mass appears to be necessary in many animals before weight maintenance becomes spontaneously established. It is proposed, therefore, that changes in the adipose tissue mass, along with shifts in the range in which glycogen levels are maintained, and other adaptive changes, contribute to bring about rates of fat oxidation commensurate with a diet's fat content. This impact of dietary composition on body composition may be a factor contributing to the increased incidence of obesity in affluent populations consuming diets with a substantial fat content.