An increase in the incidence of aneuploidy is well documented with increasing maternal age, in particular in human females. Remarkably, little is known regarding the underlying molecular basis for the age-associated increase in aneuploidy, which is a major source of decreased fertility in humans. Using mouse as a model system we find that eggs obtained from old mice (60–70 weeks of age) display a 6-fold increase in the incidence of hyperploidy as assessed by chromosome spreads. Expression profiling of transcripts in oocytes and eggs obtained from young and old mice reveals that ∼5% of the transcripts are differentially expressed in oocytes obtained from old females when compared to oocytes obtained from young females (6–12 weeks of age) and that this fraction increases to ∼33% in eggs. The latter finding indicates that the normal pattern of degradation of maternal mRNAs that occurs during oocyte maturation is dramatically altered in eggs obtained from old mice and could therefore be a contributing source to the decline in fertility. Analysis of the differentially expressed transcripts also indicated that the strength of the spindle assembly checkpoint is weakened and that higher errors of microtubule–kinetochore interactions constitute part of molecular basis for the age-associated increase in aneuploidy in females. Last, BRCA1 expression is reduced in oocytes obtained from old females and RNAi-mediated reduction of BRCA1 in oocytes obtained from young females results in perturbing spindle formation and chromosome congression following maturation.