To further define requirements for embryo development without the assistance of complex media, serum, or coculture, the effects of 5% and 20% O₂ concentrations, superoxide dismutase (SOD) (experiment 1), and taurine (experiment 2) were tested in a single culture medium (KSOM modified with 2.5 mM HEPES). Bovine embryos were produced in vitro maturation (IVM) and in vitro fertilization (IVF) of oocytes. After fertilization, embryos were cultured in 5% O₂:5% CO₂:90% N₂ at 39°C for the first 40–44 h; then embryos with 4 to 8 cells were cultured in different treatments for another 6 days. Percentage development subsequently was based on this number. In experiment 1, more blastocysts were produced in 5% O₂:5% CO₂:90% N₂ (37%) than in 20% O₂:5% CO₂:75% N₂ (18%; p > 0.01). Blastocyst development with 0, 300, and 600 U/ml of SOD, respectively, was 26%, 26%, and 30% (p < 0.05). In experiment 2, more blastocysts were formed in 5% O₂ (39%) than in 95% air (30%; p > 0.05). Also, with 20% O₂ and 0, 7, and 14 mM taurine in the medium, blastocyst formation was 18%, 35%, and 36% (p > 0.05). However, in 5% O₂, 7 and 14 mM taurine was not beneficial (p = 0.99), as 39% blastocysts were formed with no taurine. In conclusion, results from both experiments demonstrated that 5% O₂ was superior to 20% O₂ for development of IVM/IVF bovine embryos. SOD had no effect in either 5% or 20% O₂. However, 7 mM and 14 mM taurine counteracted the negative effect of 20% O₂. The best culture system tested here was simple; it resulted in sufficient blastocyst formation that it can serve as an excellent base for further studies of the requirements of embryos.