Breast cancer (BCa) was induced in vivo in female rats with 7,12-dimethylbenz(a)anthracene (DMBA). Two main questions were addressed. Firstly, would the carcinogenesis be accompanied by oxidative stress as signalled by superoxide dismutase, glutathione peroxidase, malondialdehyde and total nitrate? Secondly, would treating the rats additionally with a blocker of voltage-gated sodium channel (VGSC) activity, shown previously to promote BCa progression, affect the oxidative responses? The DMBA-induced increases in the antioxidant systems were completely blocked by the VGSC inhibitor RS100642, which also significantly prolonged the lifespan. We conclude that VGSC inhibition in vivo can significantly protect against oxidative stress and improve survival from tumour burden.