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Dietary fat is one of the most investigated nutrients in relation to breast cancer. Whereas high dietary fat favours oxidative stress and cancer promotion/progression, reduced fat intake is associated with lower recurrence rates and longer survival. However, most of the studies related to the role of dietary fat in breast cancer have been performed using rodent models, wherein these animals are fed with percentage levels of dietary fat which are very far from their natural/physiological intake. We analysed the influence of several dietary fats with a normolipidic content (4%) on circulating oxidative stress parameters (total antioxidant capacity, lipid peroxidation and protein oxidation), non-enzyme (GSH and GSSG) and enzyme (superoxide dismutase, catalase and glutathione peroxidase) antioxidant defence systems and hormonal status in Wistar rats with mammary tumours chemically induced by N-methyl-nitrosourea. We used extra virgin olive oil (EVOO), refined sunflower oil (SO), oleic acid-enriched sunflower oil (OAESO) and a commercial diet with soybean oil as the source of dietary fat. No changes could be found in the carcinogenesis parameters in this animal model in relation to the dietary fat intake. However, histological analysis showed differential effects of each dietary fat on acinus formation, nuclear atypia and mitotic index. In the same way, no changes were found in oxidative stress parameters independently of the source of dietary fat, but changes were found in non-enzyme and enzyme antioxidant defence systems in relation to the type of dietary fat. Also, different estradiol and progesterone levels were found in animals with breast cancer in relation to the dietary fat type. Although oxidative stress biomarkers were not able to demonstrate the existence of an altered redox status in animals with mammary tumours, particular changes in enzyme and non-enzyme antioxidant defence systems and in the hormonal status could be responsible for the histopathological differences found as a consequence of feeding different dietary fats.