Placental oxidative stress has been suggested as a key factor in early pregnancy failure. Abnormal placental development limits success in pregnancies obtained by somatic cell nuclear transfer (SCNT). Malondialdehyde (MDA) content, an index of oxidative stress, and superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX) activities were determined in bovine extraembryonic tissues of SCNT or artificial insemination (AI) conceptuses. Chorionic tissues of SCNT and AI conceptuses show no difference in MDA content at day 32 of pregnancy. MDA content in chorionic tissues of SCNTand AI conceptuses decreased from day 32 to 62 of pregnancy. MDA content was lower in chorionic tissues of SCNT conceptuses than that in chorionic tissues of AI conceptuses at day 62 of pregnancy. SOD1, SOD2 and GPX activities in chorionic tissues of SCNT conceptuses were not different from those in chorionic tissues of AI conceptuses at both gestational ages. CATactivity in chorionic tissues of SCNT conceptuses was lower at day 32, and it was higher at day 62 of pregnancy than that in chorionic tissues of AI conceptuses. CAT and GPX activities increased in chorionic tissues of SCNT conceptuses with gestational age. SOD1 activity decreased while that of SOD2 and GPX increased in chorionic tissues of AI conceptuses with gestational age. At day 62 of pregnancy, MDA content and enzyme activities in cotyledonary tissues were not different between AI and SCNT conceptuses. Different antioxidant mechanisms may operate within the chorion of AI and SCNT conceptuses. Further experiments are required to elucidate this point.