Placental dysfunction leads to foetal damage, which jeopardises the exchange between the maternal and foetal systems. We evaluated the effects of tumour growth on the activity of antioxidant enzymes and oxidative stress in placental tissue and cell culture from tumour-bearing pregnant rats compared to non-tumour-bearing pregnant rats that were ascitic fluid injected. Ascitic fluid is obtained from Walker tumour-bearing rats and contains a cytokine called Walker factor (WF), which is a molecule similar to proteolysis-inducing factor (PIF), and induces changes in protein metabolism and oxidative stress. Pregnant Wistar rats were distributed into control (C), tumour-bearing (W) and ascitic fluid injected (A) groups and were sacrificed on days 16, 19 and 21 of pregnancy to analyse the profile of enzyme activities (glutathione-S-transferase (GST), catalase (CAT), alkaline phosphatase (AP)) and malondialdehyde (MDA) content in placental tissue. Meanwhile, placenta samples from all groups were obtained on day 21, placed in primary culture and treated with WF for 72 h. The presence of tumour or ascitic fluid reduced the protein content of the placental tissue. On day 16 there was a significant reduction in AP activity in W rats, and on day 19, CAT activity and MDA content significantly increased. These results indicate that the presence of cancer decreased antioxidant enzyme capacity in the placenta, increasing the amount of oxidation in these cells, which may contribute to irreversible placental damage and compromisefoetal development. WF treatment induces similar changes in placental cells in primary culture, resulting in less cell viability and increased oxidative stress. These results indicate that WF, provided by the tumour or inoculation of ascitic fluid, has negative effects on placental homeostasis, which impairs foetal health.