SummaryJuvenile and adult male and female Swiss mice in metabolism cages were fed one of four specially-formulated, pelleted diets containing respectively 8% saturated vegetable fat, 8% soya oil, 8% olive oil and 2 % soya oil. The identities of the diets were hidden from the experimenter. Subjects were individually housed in metabolism cages and their consumption of food, growth and eliminative activities were measured. Clearly, these non-isocaloric diets differed in palatability, producing complex effects on growth as well as metabolic measures. Many indices were influenced by age, sex, and the duration of dietary exposure. Interactions between factors were common. Dietary fats appear to have subtle effects on the physiology and behaviour of rodents and may account for some differences between studies.