Résumé : As shown previously by several authors, cycloheximide inhibits progesterone‐induced maturation. However, normal maturation can be obtained, in Xenopus, after a 5 h treatment with cycloheximide (10–20 μ/ml), followed by extensive washing. Under these conditions, protein synthesis is still 50% inhibited. When X.laevis oocytes are continuously treated with a progesterone and cycloheximide mixture, they undergo a degenerative process which has been termed ‘pseudomaturation’: the germinal vesicle membrane breaks down, but the chromosomes do not condense and the nucleoli do not disappear completely. Progesterone‐induced maturation was not arrested by fusidic acid in Rana pipiens, even after micro‐injection. Actinomycin D (10–20 μg/ml) speeds up maturation in both R. pipiens and X. laevis. Microinjection of α‐amanitin into control R. pipiens oocytes does induce maturation in a few cases. It is concluded that one of the effects of progesterone might be a repression of RNA synthesis. Successive treatments with cycloheximide and actinomycin D failed to induce maturation, but often produced ‘pseudomaturation’ in X. laevis. Injection of a cytoplasmic extract (centrifuged homogenate) from X. laevis eggs which have undergone maturation into recipient oocytes of the same species induces ‘pseudomaturation’ and strongly inhibits protein synthesis. If the membranes which surround the egg are eliminated before homogenization, true maturation is obtained. The membranous material apparently releases factors which exert a negative effect on maturation and protein synthesis. Homogenates from eggs which have not been treated with progesterone do not, after injection, induce ‘pseudomaturation’ and have little effect on protein synthesis. In contrast with the findings of Ecker and Smith (1971)11 on R. pipiens, protein synthesis in X. laevis, after a short stimulation, drops considerably (50%) during progesterone‐induced maturation. Copyright © 1973, Wiley Blackwell. All rights reserved