BACKGROUND: It has already been shown that melatonin is an antitumoral molecule that affects malignant cells via some mechanisms. The benefit played by this hormone on cancer is due to its antioxidant effects. OBJECTIVE: This study aimed to evaluate the preclinical effects of melatonin in mice with the Ehrlich ascites tumor. METHODS: Twenty Balb/c male mice with Ehrlich tumor were treated with different melatonin doses. Their inflammatory and oxidative stress were accessed by gene expression. Hepatotoxicity and hematological parameters were also evaluated through biochemical analyses. Animal welfare was analysed weekly from the categories guided by the NC3Rs. RESULTS: Gene expression analyses have shown that only Tnfα and Sod1 were expressed in all groups studied. Only the M-3 group showed increased Tnfα expression compared to the control. All groups treated with melatonin showed decreased Sod1 expression compared to the control. No signs of hepatotoxicity were caused by any of the melatonin doses used in the treatment. CONCLUSION: In animals with Ehrlich´s tumor treated with melatonin, a decrease in oxidative stress, an amelioration in welfare and in cognitive tasks could be observed, even if the treatment has not reduced the size of the tumor itself. In parallel with the already patented use of melatonin in the treatment of sleep disorders or chronic kidney disease, our results propose its use to improve the general well-being of breast cancer patients.