To obtain information about the effects of insulin on the development of bovine embryos, fertilized bovine embryos in vitro were cultured in a chemically defined, protein-free medium: modified synthetic oviduct fluid (mSOF) supplemented with 1 mg/ml polyvinyl alcohol (PVA). The percentage of embryos reaching the morula stage was not affected by the addition of 5 micrograms/ml insulin alone, but insulin showed a beneficial synergistic effect with amino acids (AAs) on the embryonic development to morula. Supplementation of insulin (1-100 micrograms/ml) with mSOF+AAs did not improve the percentage of embryos developing to the blastocyst stage. However, a significant increase in cell numbers of blastocysts was observed with the addition of 1 mM glucose and 1 or 10 micrograms/ml insulin from the morula stage. These results show that insulin has a beneficial effect on the preimplantation development of bovine embryos in the presence of AAs and/or glucose, and suggest that insulin improves embryonic development by stimulating AAs transport and/or glucose uptake.