The effect of two concentrations of oxygen on the development of bovine embryos was compared using two separate co-culture systems. In Experiment I, bovine oocytes were matured and fertilized in vitro and were then co-cultured for 7 days in 20 mul drops of M199 with 10% fetal calf serum containing oviduct cells. When cultures were performed in an atmosphere of 5% CO(2) in air (20% O(2)) or in a mixture of 5% CO(2), 5% O(2) and 90% N(2) (5% O(2)), 22 of 179 (12%) and 56 of 179 (31%) zygotes developed to or beyond the late morula stage (P<0.0001), respectively. After freezing, thawing and 48 hours of additional culture, 2 of 21 (10%) and 18 of 53 (34%) embryos were judged viable (P<0.001) within the respective treatment groups. In Experiment II, zygotes produced by the same means were co-cultured in 0.5 ml of M199 containing 10% fetal calf serum with monolayers of buffalo rat liver (BRL) cells. In 20% O(2), 51 of 177 (29%) zygotes developed into viable embryos, while in 5% O(2) only 9 of 177 (5%) were judged viable after 7 days of culture (P<0.0001). Post-freezing survival rates were 53% and 67% for embryos from the two respective oxygen concentration treatment groups. The transfer of 20 Grade 1 frozen/thawed embryos produced by co-culture with BRL cells produced six pregnancies (30%). These experiments show that the critical effect of oxygen concentration on embryo development in vitro and the ability of embryos produced by in vitro procedures to survive freezing can be influenced by the type of culture system employed.