Mouse demi-embryos that developed from bisected morulae were transferred to recipients. The eu-blastocysts (distinct inner cell mass and well-developed trophectoderm) contained cells equal to 51% of the controls that developed from zona-free morulae. The rate of decidual cell reaction induced by the eu-blastocysts was not significantly different from that of the controls, but the size of the deciduum containing the egg cylinder was significantly smaller on Day 5.5 of pregnancy (P<0.001). A significant increase in embryonic loss was observed from Day 7.5 to Day 9.5 in the eu-blastocysts (P<0.05), while the controls exhibited no significant difference. Although the embryos from the eu-blastocysts showed retardation of developmental stages and decreased size, they attained normal stages and size regulation up to 90% of that of the control on Day 10.5. The pseudo-blastocysts (poorly developed inner cell mass enclosed by trophectoderm) contained cells equal to 25% of those of the controls and showed less than a 10% developmental rate to the egg cylinder stage. The trophectodermal vesicles (no enclosed cells) and nonintegrated forms (disorganized clusters of cells) contained cells less than 18% of those of the controls. They showed lower rates of decidual cell reaction than those in the controls (P<0.05), and no egg cylinder was found in the deciduum. The results indicate that a severe decrease in the number of embryonic cells affects the regulation of embryonic development and decidual cell reaction in the uterus.