Summary The responses of two-cell and eight-cell mouse embryos to four specific metabolic inhibitors were determined, as a means of studying nucleic acid and protein syntheses in preimplantation embryos. The embryos were grown in vitro, in various concentrations of mitomycin C, actinomycin D puromycin, or fluorophenylalanine, for two or three days, and dose-response lines were determined for each inhibitor at both stages. Actinomycin D, which blocks RNA synthesis, was able to block development of embryos in very low concentrations, indicating that the synthesis of some RNA, probably of the messenger or transfer type, is required for the normal progression of cleavage divisions. Blocking protein synthesis with puromycin also interfered with the cleavage process, suggesting that some protein involved with the mitotic apparatus must be synthesized at an early stage. Some differences were noted between the responses of two-cell and eight-cell embryos, which indicate that changes in metabolic processes, possibly involving the rate of turnover of protein, are occurring during these early developmental stages.