Mouse uteri and embryos from 12 hours to seven days after mating were examined for RNA and glycogen following specific histochemical staining. Because of their greater basophilia, calls present throughout the endometrium which seem to be metrial gland cell precursors could be distinguished as early as two and three-quarter days. At this time, the uterus is just beginning to undergo “sensitization to decidualization.” By seven days, these precursor cells and metrial gland cells proper have disappeared from all regions of the endometrium except that of the future decidua basalis. Glycogen is present in cells which may be the metrial gland cell precursors, plasma cells and polymorphonuclear leucocytes as well as in the lumens and cells of the uterine glands at three and one-half days. Between four and one-fourth and five and one-half days, glycogen is found in the primary decidual zone. By six days, there is a heavy deposit in the “glycogenic areas.” At seven days, many of the metrial gland cells in the future decidua basalis are PAS-positive. These findings are discussed and the possible origin of the metrial gland cells from a cell of the lymphocytic series is suggested.