The activity of glucose 6-phosphate dehydrogenase was determined in mouse embryos during the first 5 days of development from the time of ovulation up to implantation. 2. The activity decreased from 1.39 to 0.19mmumoles of NADP reduced/hr./embryo from ovulation to implantation. 3. The specific activity of the embryo on the first day was higher than specific activities reported for most other tissues.