The effects of maternal aging on the configuration of chiasmata, formation of univalents, and segregation of first meiotic (MI) chromosomes were investigated in young (5-8 mo) and old (16-19 mo) Chinese hamsters. Primary oocytes were collected only from mature follicles approximately 10 h before ovulation, and secondary oocytes were obtained from the oviducts 5 h after spontaneous ovulation. The average number of chiasmata per oocyte was significantly smaller in aged hamsters than in the young hamsters (P less than 0.001). Terminal chiasmata were found more frequently in the former group than in the latter one (P less than 0.001). These results coincided well with findings in the mouse. Since the 11 meiotic chromosomes could be divided into four morphologically distinguishable subgroups, it was possible to determine whether the same bivalent forming univalents at MI actually underwent nondisjunction in the following meiotic division. The incidence of both MI oocytes with a univalent pair and aneuploid MII oocytes due to first meiotic nondisjunction was significantly higher in the aged group than in the young group (P less than 0.01) and P less than 0.05, respectively). However, univalents occurred almost exclusively in the smallest metacentric chromosome group (96%), whereas nondisjunction took place nearly equally in each chromosomal subgroup. These results clearly showed that there was no correlation between the univalents seen at MI and nondisjunction during the first meiotic division.