In recent decades, several studies have been conducted to clarify the role of free radicals in pathophysiological processes such as aging, inflammation, and cancer. Mammary cancer is the most common neoplastic disease of adult female dogs, and studies have shown that the size of the tumor, involvement of lymph nodes, and histological grade constitute as negative prognostic factors in advanced mammary cancer, but there are few data about early stages of the disease even in human beings. This study aimed to evaluate the oxidative metabolism using the product concentrations of advanced protein oxidation (AOPP) and the antioxidant activity by reducing iron (FRAP) of bitches with mammary carcinoma in early stage. Twenty intact female dogs with histopathological diagnosis of mammary carcinoma grade 1 and clinical stage I according to the WHO (T1N0M0 - mammary tumor <3 cm without regional lymph node involvement and/or evidence of distant metastases) and ten intact healthy female dogs were used as a control group. The use of animals was approved by the Ethics Committee on Animal Use (010289/14). Data were analyzed by one-way analysis of variance followed by t-test. Serum concentrations of FRAP and AOPP of the mammary cancer group in early stage of the disease (1586±157.5 and 15.8±3.37 mmol/L, respectively) were significantly higher than healthy dogs (711±99 and 5.15±0.07 mmol/L; p=0.0045 and p=0.007, respectively), showing that there are already damages in lipids and proteins mediated by oxidation even in the early stage of canine mammary carcinoma (acknowledgement to FAPESP for financial support).