Growth factors are known to play important roles in cellular proliferation and differentiation. However, little information is available concerning their roles in the earliest stages of mammalian development. The effect of physiologic levels of insulin, insulinlike growth factor-I, and insulinlike growth factor II (IGF-I and -II) on DNA, RNA, and protein synthesis in preimplantation stages of the mouse are described in this study. Quantitative studies of the incorporation of labeled thymidine, uridine, and methionine into trichloroacetic acid-insoluble material by different developmental stages of preimplantation mouse embryos labeled in vitro, indicate that physiologic levels of insulin stimulated DNA, RNA, and protein synthesis with significant effects observed first at the morula stage of development. In contrast, neither IGF-I nor IGF-II stimulated DNA, RNA, or protein synthesis to a significant degree under the same experimental conditions. These results suggest a functional role for insulin at the earliest stages of mammalian embryogenesis.