Inactivation of the X-linked gene in the female embryo is one of the major events during mammalian early embryogenesis. Before this inactivation, enzyme activity encoded by the X-linked gene is different between male and female embryos. In the present study, we demonstrated a possibility that there may be a different response to specific culture conditions in vitro between male and female early embryo. Glucose-6-phosphate dehydrogenase (G6PD) is one of the X-linked enzymes and its activity is semi-quantitated by brilliant cresyl blue (BCB). In experiment 1, the relationship between semi-quantitated G6PD activity and the sex ratio of the embryos was examined. In experiment 2, the relationship between semi-quantitated G6PD activity and the developmental competence of embryos was examined. No relationship was found between G6PD activity and sex in 8-cell, 16-cell and blastocyst stage embryos. However, G6PD activity was high in female embryos and low in male embryos at the morula stage. When morula stage embryos were categorized by BCB and cultured under high oxygen tension (5% CO2, 95% air), the developmental competence of embryos having high G6PD activity was higher than those having low enzyme activity. However, when the categorized morula stage embryos were cultured under low oxygen tension (5% CO2,5% O2, 90% N2), there was no difference in the developmental rates among the categories. These results indicate that G6PD activity is high in female morula stage embryos compared with that in male embryos, and that the activity of this enzyme strongly affects the developmental competence of morula stage embryo when they are cultured under high oxygen tension.