The aim of the present study was to reveal the effects of in vitro aging of mouse oocytes without cumulus cells on morphological alteration of their metaphase II (MII) spindles, ability to be fertilized in vitro and the developmental competence of the resulting embryos. To observe the MII spindles by immunofluorescence staining, oocytes without cumulus cells were divided into four groups: a non-aged control and three others aged for 10, 15 or 25 h. Although the incidence of morphologically normal spindles was significantly lower in the 25 h group, no significant increase of chromosome misalignment was observed in any of the aged groups (P < 0.05). For the in vitro fertilization (IVF) experiments, denuded oocytes were aged for 15, 16.5 or 18 h and the resulting normal zygotes with a second polar body were cultured in vitro for 120 h to assess their embryonic development. The fertilization rate was significantly lower only in the 18-h aged group. The rates of blastocyst formation were significantly lower in all aged oocyte groups, compared with non-aged controls, and blastocysts derived from aged oocytes had lower total cell numbers. Therefore, a significant decline of developmental competence appears in cumulus cell-removed oocytes aged for more than 15 h, even if they retain fertilizability.