Rex1/Zfp42 is a nuclear protein that is highly conserved in mammals, and widely used as an embryonic stem (ES) cell marker. Although Rex1 expression is associated with enhanced pluripotency, loss-of-function models recently described do not exhibit major phenotypes, and both preimplantation development and ES cell derivation appear normal in the absence of Rex1 . To better understand the functional role of Rex1 , we examined the expression and localization of Rex1 during preimplantation development. Our studies indicated that REX1 is expressed at all stages during mouse preimplantation development, with a mixed pattern of nuclear, perinuclear, and cytoplasmic localization. Chromatin association seemed to be altered in 8-cell embryos, and in the blastocyst, we found REX1 localized almost exclusively in the nucleus. A functional role for Rex1 in vivo was assessed by gain- and loss-of-function approaches. Embryos with attenuated levels of Rex1 after injection of zygotes with siRNAs did not exhibit defects in preimplantation development in vitro. In contrast, overexpression of Rex1 interfered with cleavage divisions and with proper blastocyst development, although we failed to detect alterations in the expression of lineage and pluripotency markers. Rex1 gain- and loss-of-function did alter the expression levels of Zscan4 , an important regulator of preimplantation development and pluripotency. Our results suggest that Rex1 plays a role during preimplantation development. They are compatible with a role for Rex1 during acquisition of pluripotency in the blastocyst.