Changes in RNA localization and concentration in mouse uteri taken from one-half to seven and one-quarter days after mating were visualized as changes in basophilia following staining by a histochemical method specific for RNA. The basophilia of the luminal epithelium falls abruptly between one-half and two days and is still lower at two and three-quarter days. By 2 and three-quarter days, however, that of the stroma and especially that of the subepithelial endothelial cells is increasing. A further slight increase is found at three and three-quarter days and by four days, a band of subepithelial stromal cells, wider in the antimesometrial than in the mesometrial portion of the uterus, is highly basophilic. At four and one-quarter days, when a distinct decidual reaction is evident, cells in the primary decidual zone are less basophilic than at four days. During the remaining period of observation, cells in different regions of the uterus show increased and often, later, decreased basophilia. It was concluded that these results are in complete accord with those from a recent biochemical study of changes in RNA in the intact rat uterus during the interval from induction of pseudopregnancy through deciduoma formation if the difference in timing in the two species is taken into account.