Hypothesis. Renin–angiotensin system (RAS) has been considered not only as a regulator of systemic volume and electrolyte balance but also has been recently involved in various pathological processes such as cancer. In the etiology of breast cancer, dietary factors have been analyzed and especially the influence of dietary fat has been studied, but the underlying mechanisms remain unclear. In this study, we analyzed RAS-regulating enzymes in serum of rats with N-methyl nitrosourea (NMU)-induced breast cancer fed with different diets. Study Design. Four groups of rats were injected intraperitoneally with 3 doses of 50 mg/kg body weight of NMU at different days after birth and were fed with an AIN-93 commercial diet or AIN-93 diets with 4% fat constituted respectively by extra virgin olive oil, refined sunflower oil, and refined sunflower oil enriched to 50% with oleic acid. Method. After sacrifice, blood and tumor samples were collected by spectrophotometric determinations of RAS-regulating enzymes in plasma and histopathology studies. Results. We show that the type of dietary fat does not influence latency period, incidence of animals with tumors, incidence of mortality, or tumor yield per rat. However, changes were observed in tumor volume and the histopathology. The type of dietary fat also differently modified the enzymes involved in RAS regulation. Conclusions. It might suggest that one of the mechanisms by which dietary fat affects breast cancer is the modification of the RAS system, which may be consider as a new target for integrative therapies.