Embryo development depends on maternal and embryonic factors that may regulate genetic programs in early development. Effects of growth factors on proliferation, differentiation and morphogenesis along embryogenesis have been documented. However, studies have not established the role of growth factors in the preimplantational period. The purpose of this study was to investigate the possible effects of growth factors and embryo density on mouse preimplantation development in vitro. Two- and eight-cell CF-1 embryos were cultured individually or in groups of ten in HTF medium, alone or with EGF, TGF-beta 1 and IGF-I. Cleavage rate varied greatly with growth factors and increased significantly when eight-cell embryos were cultured in groups. On the other hand, when two-cell embryos were cultured in groups, the cleavage rate was slower than that obtained when embryos were individually cultured. The differentiation rate increased significantly in two-cell embryos cultured in groups (p < 0.05). EGF, TGF-beta 1 and IGF-I increased differentiation rates significantly in two-cell embryos individually cultured for 68 hours. The combination of EGF and TGF-beta 1 increased the differentiation rates significantly. The other combinations were not effective in modifying this parameter. Hatching rates increased in embryos cultured in groups (p < 0.05). TGF-beta 1 decreased this parameter significantly in two- or eight-cell embryos cultured in groups (p < 0.05). The data described in this report suggest that preimplantational mouse embryos produce some factor or factors that enhance its development, specially the differentiation and hatching rates. However, a functional role for polypeptide growth factors during preimplantational development has to be determined.