Mouse ovarian oocytes at various stages of maturation were collected and inseminated with preincubated (capacitated) spermatozoa. Within 1 hr of insemination, sperm penetration into the perivitelline space was observed in oocytes at any stage of maturation. The proportion of oocytes with spermatozoa in the perivitelline space and in the vitellus 1 hr after insemination increased as the nuclear maturation of oocytes advanced to Metaphase I-Telophase I. In the oocytes penetrated earlier than Metaphase I, the swollen sperm head changed to condensed chromatin and, in some, the chromatin changed into a spindle-like form. The proportion of oocytes with well-formed pronuclei increased markedly when oocytes were inseminated 10 hr after the injection of hcg at the stage of Metaphase I-Telophase I. It appears that the reactivity of oocytes to spermatozoa occurred gradually after the breakdown of the germinal vesicle and their fertilizability was acquired just before, or at the time of, completion of the first maturation division.