To study the effects of insulin and insulin-like growth factor-I (IGF-I) on early development of bovine embryos, one-cell embryos were cultured in a chemically defined, protein-free medium: modified synthetic oviduct fluid. The addition of 10 μg/ml insulin or 20 ng/ml IGF-I increased the percentage of morulae at Day 5 (5 days after insemination). The cell number of blastocysts which developed after 2 days culture of morulae obtained at Day 5 was increased by supplementation with insulin or IGF-I. The combination of insulin and IGF-I had neither additive nor synergistic effect on the embryonic development. These results clearly show that insulin and IGF-I improve the in vitro development of bovine embryos.