Oct-4 is a key transcription factor in the control of early embryonic development and maintenance of a pluripotent cell population. Variation in Oct-4 expression patterns during embryo development have been reported among species, and have been related to the time of placental development in those species. This study was conducted to investigate Oct-4 expression pattern during early embryonic development in the horse, a species with relatively delayed placentation. In vitro-produced embryos were obtained from in vitro-matured oocytes via fertilization by intracytoplasmic sperm injection. Ex vivo blastocysts were recovered from mares that had been artificially inseminated. Oct-4 status was determined by immunocytochemistry; photomicrographs were taken at 4 standardized settings to aid in qualitative comparison of the amount of fluorescence. A total of 106 oocytes and embryos were evaluated. Immature oocytes showed Oct-4 expression in the nucleus and cytoplasm, as did early-cleaved embryos (2 to 5 cells, 1 to 2 days). Oct-4 expression in embryos at 3 to 4 days (6 to 12 cells) decreased and was restricted to the cytoplasm. From 5 to 6 days (15 cells to morulae), Oct-4 intensity increased and was exclusively found in the nuclei. In vitro-produced blastocysts (7 to 8 days) expressed Oct-4 equivalently in the trophectoderm and inner cell mass nuclei; culture for 2 to 3 more days (10 to 11 days) did not alter Oct-4 expression. However, when in vitro-produced blastocysts were transferred to the uteri of mares and recovered after 2 to 3 days (IVP-ET), the embryos showed strong expression of Oct-4 within the inner cell mass and limited expression in the trophectoderm, and a similar pattern was seen for ex vivo-recovered embryos. In bigger embryos (such as a 1779-m ex vivo embryo and a 1121-m IVP-ET embryo), the trophectoderm lost staining completely. These results suggest that Oct-4 expression is present in both nucleus and cytoplasm in equine oocytes and early-cleaved embryos as a result of maternal mRNA accumulation. Oct-4 protein decreases over the first few days of embryonic development as these stores are used. The shift to greater expression, in the nucleus only, during further embryo development suggests embryonic genome activation. Oct-4 expression in the trophectoderm of in vitro-produced blastocysts was different from that in blastocysts that had been exposed to the uterus (both ex vivo and IVP-ET); this indicates that differentiation of the trophectoderm is dependent upon factors present in the uterine environment. The Oct-4 expression in the trophectoderm of in vitro-produced equine blastocysts thus appears to be an artifact due to in vitro culture; this finding may be applicable to the reported patterns of Oct-4 expression in embryos of other species. This work was supported by the Link Equine Research Endowment Fund, Texas A&M University.