Human oocytes exposed to capacitated spermatozoa in vitro when at metaphase of the 1st meiotic division (metaphase I) were not penetrated, even though some subsequently progressed to metaphase of the 2nd meiotic division (metaphase II). When the non-penetrated oocytes that had reached metaphase II during the incubation with spermatozoa were freed from the zona pellucida and reinseminated, two or more pronuclei developed in most of them. By contrast, no penetration was observed when the oocytes were reinseminated in the zona-intact state. When compared with metaphase II oocytes, metaphase I oocytes had a similar zona-binding capacity for spermatozoa, but fewer spermatozoa were found within the zona. These results indicate that the zona pellucida of human oocytes undergoes important maturational changes during the transition from metaphase I to metaphase II. Ultrastructural and previous histochemical findings suggest that these changes involve secretions from both the oocyte and cumulus cells and that the increased zona resistance at metaphase I may be due to relative insufficiency of cumulus cell-secreted 'softening' factors. If the integrity of the cumulus oophorus is disturbed at this stage, this condition becomes irreversible.