Design of diets for the study of dietary influences in mammary gland carcinogenesis requires attention to several questions: (1) Do the diets satisfy the nutritional needs of the animal under the conditions of the experiment, and are they palatable? (2) Does the protocol include determination of feed intake (if indicated) and of achievement of the desired level of nutrient deficiency, adequacy, or excess? (3) Are there potentially confounding nutrient interactions or nutrient effects or physiological or pathological responses that must be considered? The particular sensitivity of mammary gland tumorigenesis to intake of fat and calories and to body weight gain must be considered and controlled for in all experiments.