To study the effects of insulin and insulin-like growth factor-I (IGF-I) 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. Dose-response studies showed that insulin (0.5 to 10 microg/ml) and IGF-I (2 to 200 ng/ml) stimulated the development of bovine embryos to the morula stage 5 d after in vitro fertilization. The addition of 0.5 microg/ml insulin or 2 ng/ml IGF-I to the mSOF had beneficial effects on embryonic development to the morula stage in the presence of amino acids, but insulin and IGF-I did not affect the development of bovine embryos to the morula stage in the absence of amino acids. The antiIGF-I receptor antibody (alphaIR-3) completely blocked the stimulation of development to the morula stage by insulin and IGF-I. These findings suggest that the stimulation of embryonic development by insulin and IGF-I is mediated through the IGF-I receptor.