Analysis of macromolecular synthesis by preimplantation mammalian embryos in culture has revealed that activation and expression of the embryonic genome occurs in early development. The inhibition of cleavage in mouse embryos by actinomycin D (Mintz, 1964; Skalko & Morse, 1967; Monesi, Molinaro, Spalletta & Davoli, 1970) suggests that RNA transcribed from the embryonic genome may be necessary for continued development as early as the 2-cell stage. Because actinomycin D has been reported to exert effects not clearly related to transcriptional inhibition (Manes, 1975), its role in the suppression of embryonic development should be regarded with some caution. In contrast, the mushroom toxin, α-amanitin, is a selective inhibitor of RNA polymerase II both in vitro and in vivo (Stripe & Fiume, 1967; Tata, Hamilton & Shields, 1972; Hadjiolov, Dabeva & Mackedonski, 1974). Preliminary reports have suggested that α-amanitin may inhibit cleavage of mouse embryos in culture (Golbus, Calarco & Epstein, 1973; Warner & Versteegh, 1974), although only a relatively narrow range of α-amanitin concentrations has been tested and only studies of embryos exposed at the 2-cell stage have provided sufficient data to establish statistical significance. To verify the inhibitory action of this drug and to establish dose–response relationships essential for analyses of messenger RNA metabolism, we have investigated the effects of α-amanitin upon cleavage and blastulation of mouse embryos explanted into culture at several stages of development before implantation.