In vitro bovine embryos production and quality was evaluated in two culture systems, which utilize different oxygen tension. After IVM/IVF presumptive zygotes were cultured in either one of the two systems. The culture systems evaluated were-high O2: SOFaaci medium and culture for 7 d under 5% CO2 in air, at 39 degrees C in the presence of cumulus cells (control); low O2: SOFaaci medium and culture for 7 d under 5% CO2 and 5% O2 at 39 degrees C. In low O2 system the zygotes were denuded by successive pipetting before being transferred to culture medium, while in the high O2 zygotes kept the cumulus cells that remained after IVF. Cleavage rates were evaluated 48 h post-insemination (hpi) and the blastocyst rates at D6 and D7 post-insemination (pi). From both groups a total of 94 expanded blastocysts, from D7 of culture, were fixed and stained with aceto-orcein to evaluate cell numbers. Seven pools of 15 embryos from each treatment were frozen for gene expression evaluation. The abundance of transcripts for genes related to oxidative stress, superoxide dismutase (Mn-SOD), catalase, gluthatione peroxidase (GPX) and for embryo quality, interferon-tau (IFN-tau) were determined using a semi-quantitative RT-PCR. Cleavage rate was similar (P>0.05) for both groups. The blastocyst rate at D6 pi was greater (P<0.05) in the group cultured under low O2 tension (37.4%) than in the high O2 tension (21.9%). However, blastocyst rate and total cell number at D7 were similar (P>0.05) between groups. No change (P>0.05) in transcript amount between treatments was observed for GPX, catalase and IFN-tau genes. However, the relative abundance of transcripts for Mn-SOD gene was greater (P<0.05) for embryos cultured in high O2 tension system. The results suggest that bovine embryos can be cultured either in SOFaaci medium under greater O2 tension in the presence of cumulus cells, or in SOFaaci medium under less O2 tension, without affecting embryo production or quality.