The long-term ingestion of a fat-rich diet caused severe obesity in adult rats. Severe obesity developed in these animals without them having to increase caloric intake. Evidence for this comes from the observation that voluntary intake averaged 36,113 +/- 410 calories/rat per 60 wk for rats eating a high-fat diet (42% of calories from fat) compared to a value of 36,125 +/- 500 calories/rat per 60 wk for those eating a diet of Purina chow. Despite a similar caloric intake, carcass fat averaged 51 +/- 1% for rats eating the fat-rich diet but only 30 +/- 1% body fat for control animals eating the low-fat diet of Purina chow. These results indicate that a fat-rich diet does not always cause rats to overeat. Further, they clearly demonstrate that severe obesity can develop in the absence of hyperphagia provided the animals eat a fat-rich diet. Finally, a correlation coefficient of r = 0.76 between body weight and lean body mass was obtained for the Purina chow-fed rats and that of r = 0.66 was obtained for the fat-fed rats.