One hundred fifty-nine human oocytes cultured from a minimum of 24 to a maximum of 53 h were studied by means of serial sectioning for high-resolution microscopy. On the basis of the morphologic characteristics displayed at the end of the culture period, 61 oocytes had remained meiotically inactive, 35 had resumed meiotic maturation and 42 had matured. In general, the fine morphologic features of the oocytes in the three groups were found to be similar to those of human oocytes at comparable stages of maturation in ovarian follicles: this demonstrates, morphologically at least, that the in vitro experiments are valid models for studying human oocyte maturation. New observations resulting from this study concern the morphological changes associated with resumption of meiotic prophase which were observed in seven oocytes, the frequent presence in immature oocytes of eccentrically located nuclei and chromosomes, and the occurrence of maturation anomalies whose incidence, however, was extremely low.