The ultrastructure of day 7 bovine blastocysts developed in vitro in either of two culture systems was compared with that of morphologically normal blastocytes collected non-surgically from superovulated cows on day 7 (Day 0 = day of insemination). The in vitro-embryos were obtained after culture of in vitro-matured and -fertilized oocytes either in a serum-free, cell-free medium (SFM, i.e. TCM 199 supplemented with BSA (10 mg/ml), insulin (5 micrograms/ml), transferrin (5 micrograms/ml) and selenium (5 ng/ml) or in a serum-supplemented medium (TCM 199 and 10% (v/v) oestrous cow serum) together with bovine oviduct epithelial cells (BOEC). Five of the 8 blastocysts developed in SFM fulfilled the criteria set for normal morphology of the in vivo-developed blastocytes. In contrast, 6 out of 8 blastocysts developed in co-culture with BOEC were classified as morphologically deviated, and only 2 reached the criteria for morphological normality. In vitro-developed blastocysts with deviated morphology showed a higher degree of cytoplasmic vacuolation, short, less developed cell-to-cell contacts between trophoblast as well as between inner cell mass (icm)-cells, less developed apical microvilli on the trophoblast and wide inter-cellular spaces. Additionally, numerous cytoplasmic vesicles, phagosomes, lipid droplets and hooded mitochondria were commonly present, both in trophoblast and in icm-cells. The results indicate that a high proportion of blastocysts developed in co-culture with BOEC were morphologically deviated compared to those cultured in medium where serum and somatic cells were replaced by BSA, insulin, transferrin and selenium.