The chromosome complement of metaphase II oocytes matured by different methods was determined. These oocytes were collected from young and old Swiss-Webster random bred and CBA inbred mice. The results from the two strains were remarkably similar. Superovulation caused no increase in aneuploidy frequencies; however, in vitro maturation resulted in increased hyperploidy rates for oocytes from younger females of both strains. Of the six possible comparisons regarding a maternal age effect, there were no significant increases in hyperploidy. The absence of a maternal age effect on the first meiotic division of oocytes, while such an effect does exist for mid-gestation mouse fetuses, suggests that the maternal age effect on aneuploidy may be due to a decreased ability to select against (abort) aneuploid embryos/fetuses as a function of increasing maternal age.