Progesterone induces in vitro, maturational events in Xenopus laevis oocytes. We report that gammexane (γ-chlorocyclohexane), an inositol analog, strongly inhibits progesterone action, providing a new tool for studying the effect of steroid on meiosis. The compound does not block the overall protein synthesis on oocytes. The antagonist effect is reversible and might take place during early stage of progesterone action since it does not abolish the activity of progesterone activated cytoplasm on maturation. It has been suggested that progesterone is effective at the surface level in Xenopus oocytes, and it is postulated that the inhibitor acts on the plasma membrane, possibly by affecting phosphatidylinositol metabolism and/or turnover.