Abstract. The present study was conducted to determine the relationship between embryonic development speed at different stages (the cleaved stage at 52 h and the blastocyst stage at 6 days post insemination) and incidences of chromosome abnormalities in in vitro produced porcine embryos. Porcine oocytes were collected from 3-6-mm ovarian follicles obtained at a slaughterhouse and matured in modified NCSU-37 medium for 44-46 h. Following in vitro fertilization with a final concentration of 1 × 10 5 sperm/ml for 3 h, all oocytes were cultured in vitro for 52 h. Day-2 (52 h after insemination) embryos were classified according to their cleaved stages into 2-cell, 3-to 4-cell, 5-to 8-cell, and >8-cell stages; these were cultured separately for additional 4 days (Day 6). The resultant Day-6 blastocysts were classified according to the morphological diameter into 3 grades: Grade A, expanded blastocysts; Grade B, expanding blastocysts; and Grade C, early blastocysts. They were then analyzed chromosomally. The 3-to 4-cell and 5-to 8-cell embryos had significantly high blastocyst development rates (46.1 and 36.9%, respectively), and these blastocysts contained significantly more cells (40.2 and 42.4 cells, respectively) than those derived from 2-cell embryos and >8-cell embryos (28.6 and 26.5 cells, respectively). The incidence of chromosomal abnormalities was significantly higher in the blastocysts derived from 2-cell and >8-cell stage embryos than in the blastocysts derived from the other stage embryos. Furthermore, the grade A blastocysts had the lowest incidence of chromosomal abnormalities (35.3%) and contained the most cells (48.7 cells). Porcine in vitro production (IVP) yielded a high blastocyst rate and an excellent embryo quality when 3-to 4-cell and 5-to 8-cell stage embryos were selected on Day 2 after insemination. The same criteria yielded a higher quality of expanded blastocysts based on the stage of embryo development and morphology. Key words: Blastocyst, Chromosomal abnormality, Embryo, In vitro production (IVP), Pig (J. Reprod. Dev. 54: [22][23][24][25][26][27][28][29] 2008) n mammals, the technology of in vitro production (IVP) of embryos, which involves in vitro maturation and fertilization of oocytes and in vitro culture of the resultant embryos, has become important for increasing the number of offspring from selected high-quality animals and for reducing the generation intervals for breed improvement in livestock [1,2]. Moreover, IVP technology has been used in the production of cloned and transgenic animals, which can be used in biomedical research [3][4][5]. In swine, although embryos have been successfully produced using the IVP system, the development rates to the blastocyst stage and to full term after embryo transfer are still low [6][7][8]. The viability of porcine embryos produced by IVP is low compared with that of in vivo-derived porcine embryos. For example, porcine embryos produced by IVP are of poor quality in terms of the number of cells compared with those produced in vivo [9].A...