Five groups of 10, 31, 49, 11 and 10 female rats, weighing on the average 110, 148, 195, 252, and 297 g, respectively, were fed a synthetic sterol free diet for 14 days, the feces being discarded for the first 7 and retained for the next 7 days. The total lipids of the feces were analyzed for unsaponifiable (UM) and digitonin-precipitable material (DM). At the end of the 14 days the length and weight of the small intestine and weight of the liver were determined when the total body weights were those indicated above. It was found that the weight and length of the intestine and weight of the liver regressed curvilinearly on the body weight, whereas the combined weight of the intestine and liver regressed linearly on the body weight. The daily endogenous excretion of sterol, calculated from the DM in the UM, regressed linearly on the body weight, on the combined weight of the liver and small intestine, and on the calories required for maintenance. The intraspecies relationships found in this study on the rat confirm those for the interspecies relationships when the data on the rat, chicken, dog, and man were analyzed and compared.