Two-cell sheep embryos and 2-4-cell and 8-cell cow embryos were cultured for 5 days in stoppered test-tubes in Synthetic Oviduct Fluid supplemented with 32 mg BSA/ml. The medium had been previously equilibrated with one of the following O2 concentrations (sheep: 0, 2, 4, 6, 8, 10, 12, 17, 20%; cow: 0, 4, 8, 12, 17, 20%). At the end of culture embryos were examined for morphology and stained to assess numbers of nuclei. Mean (+/- s.e.m.) nuclei/embryo was highest at 8% O2 for sheep embryos (23.6 +/- 3.1), 4% for 2-4-cell cow embryos (23.2 +/- 6.1) and 8% for 8-cell cow embryos (29.6 +/- 5.2). The minimum number of nuclei/embryo occurred at 20% O2 in each case (10.3 +/- 0.9, 10.3 +/- 2.7, 14.5 +/- 2.4, respectively) with similar values also recorded at 0% O2 (10.8 +/- 1.9, 16.5 +/- 6.0, 14.6 +/- 2.4, respectively). Analysis of the proportion of embryos reaching at least the morula stage demonstrated a significant quadratic component for the different oxygen concentrations for sheep (P less than 0.01) and cow (P less than 0.05) embryos. A number of sheep and cow embryos showed abnormalities, suggesting that the culture conditions require further refinement. The results confirm that, under lowered oxygen levels, development of sheep and cattle embryos can occur through the 8- to 16-cell block in a simple defined medium without somatic cell support.