A micro-Western analysis method both sensitive and quantitative enough to analyze oocytes and embryos is developed. GLUT1 protein is present in mouse oocytes and preimplantation embryos and levels are increased by fertilization and with time in ensuing embryonic development; the levels were 20-fold greater in blastocysts than in unfertilized oocytes. Similar increases were observed in glucose uptake by oocytes and embryos, suggesting that they may depend on GLUT1 expression. These results suggest that GLUT1 expression may explain a switch in substrate preference of the embryo from pyruvate to glucose during preimplantation development.