The caloric contribution of diets supplemented with sucrose, corn oil, or tallow with or without additional calcium was examined using female CD1 weanling mice. Mice were limit-fed a semi-purified diet alone or with added isocaloric amounts from sucrose, corn oil, or tallow for 28 days. In addition, diets with suppelmental fat contained either 0.60% or 1.5% calcium. Fecal fat and fecal soap excretions were greater (p<0.06) for mice fed tallow than for those fed corn oil. Mean metabolizable energy values for sucrose, tallow, and corn oil averaged 4.01, 7.96, and 8.94 kcal, respectively. Retention of digested energy from sucrose, tallow and corn oil averaged 13%, 10% and 21%, respectively. Hence, per gram of added nutrient, retained energy from tallow averaged 1.60 and that from corn oil averaged 4.11 times that of added sucrose. Retained energy from added corn oil was greater (p<0.01) than from added tallow. On a retained energy basis, the relative value for corn oil was greater and the relative value for tallow was less than the metabolizable energy ratio of fat to carbohydrate proposed by Atwater of 2.25.Added calcium depressed(p<0.01) digestibilities of both dry matter and energy with a greater(p<0.01) effect on tallow than on corn oil. These findings imply that the source of fat and calcium in the diet influence the avail-ability energy in diets and should be considered in feed formulations.