Changes in the organization of membranous structures in the amphibian oocyte cortex were studied during the process of progesterone-induced meiotic resumption. Progesterone treatment of Xenopus laevis oocytes induced short term and longer term changes in the cortical membranes. In the short term, progesterone induced a burst of endocytosis mediated through coated pits and coated vesicles. Immuno-electron-microscopic localization of progesterone suggested that the progesterone receptor, bound to its ligand, is endocytosed during progesterone-induced endocytosis. Also demonstrated was the existence of a cisternal membrane network, referred to as the primordial cortical endoplasmic reticulum, which surrounds portions of the cortical granules in oocytes. The primordial cortical endoplasmic reticulum is more highly developed in the animal hemisphere than the vegetal hemisphere. Over the long term, during the meiotic resumption, more membrane is recruited into this network to form the cortical endoplasmic reticulum observed by others in the metaphase II egg. This evidence demonstrates that the cortex serves as a site for dynamic changes in membrane organization and that the most extensive changes occur in the animal hemisphere. These data support previous observations that the animal hemisphere is better structured for sperm penetration than is the vegetal hemisphere.