To investigate whether female fertility decreases with age due to poor oocyte quality, we examined the presence of DNA fragmentation in ovulated oocytes from young, mature and aged mice. Oocytes from three age groups of female mice (7-8, 20-24 and 40-48 weeks) were retrieved from the oviducts 15 h after human chorionic gonadotrophin (HCG) injection. Oocytes from each mouse were incubated in a CO2 incubator for 0-60 h in human tubal fluid (HTF). After incubation, each oocyte was stained with the terminal deoxynucleotidyl transferase-mediated dUDP nick-end labelling (TUNEL) method. The rate of DNA fragmentation (interpreted as apoptotic changes) was significantly higher for oocytes from aged mice, and the fertilization rate was significantly lower, compared with oocytes from young and mature mice. Our results suggest that DNA fragmentation of oocytes might be one of the reasons for poor oocyte quality and lower fertility in the aged group.