Summary. Mouse blastocysts transferred to the oviducts of immature females entered a period of diapause from which they could be activated by culture in vitro or by transfer to pseudopregnant recipients. In the tract of immature females, embryos hatched from the zona pellucida, increased cell number to a maximum that is comparable to that of blastocysts delayed by ovariectomy, and some moved to the uterus. Viability of blastocysts retained in the non-progestational, immature tract remained high for 4 days but dropped after 5 or 6 days. This new method of producing a delay in implantation offers precision in determining survival and viability rates and in determining the requirements of diapausing embryos.