Hyaluronic acid (HA) is the main glycosaminoglycan present in follicular, oviductal and uterine fluids. The main functions of HA include dynamic processes that are mediated through interaction with extracellular matrix components, regulation of gene expression, cell proliferation and cell differentiation. HA increases the viscosity of solutions and also has several physiological functions, including regulation of water distribution and water-binding capacity. The addition of 6 mg HA ml(-1) to synthetic oviduct fluid (SOF; SOF-HA) culture medium on day 5 (IVF = day 0) significantly (P < 0.001) increased the viscosity of the medium in comparison with SOF culture medium containing BSA (SOF-BSA). On day 8, rate of blastocyst development in SOF-HA culture medium was significantly (P < 0.05) higher than in SOF-BSA culture medium (38.2 versus 29.3%). The number of trophectoderm cells and the total number of cells of expanded blastocysts cultured in the presence of HA were significantly (P < 0.01) higher in comparison with expanded blastocysts cultured in the presence of BSA (88.9 +/- 7.3 versus 67.6 +/- 3.0 and 130.1 +/- 10.9 versus 104.8 +/- 2.5, respectively). After freezing and thawing, the percentage of day 8 embryos that re-expanded and hatched when cultured with SOF-HA was greater than that of embryos cultured with SOF-BSA (11.3 and 10.5% versus 75.5 and 36.8%, respectively). After thawing, the ATP contents of in vivo-derived, SOF-HA and SOF-BSA expanded blastocysts were similar. The embryos cultured with HA showed less ultrastructural deviation and de-differentiation after freezing and thawing than the embryos cultured with BSA. This study demonstrates that HA improves the developmental capacity of bovine embryos under in vitro conditions and is warranted as a supplement for in vitro production of bovine embryos, particularly if they are to be cryopreserved.