The effect of maternal ageing on the meiotic rate, on chiasma and univalent frequency as well as on heteroploidy in secondary oocytes from Djungarian hamsters was examined. The frequency of hyperhaploid oocytes increased from 0.6% in young (8-14 weeks) to 2.8% in middle-aged (26-46 weeks) and reached 3.6% in the oldest females (49-75 weeks). On the basis of malsegregated bivalents per oocyte, nondisjunction occurred most often in the middle-aged group (5.42 X 10(-2) bivalents per oocyte). Hereby, the large meta- and submetacentric A-D chromosomes were preferentially involved. Furthermore, the pattern of nondisjunction was not different from that expected on the basis of chromosome length or induced by colchicine. The large A-D chromosomes did not show any alteration in chiasma or univalent frequency. Terminalized chiasmata were only detected in the E group and univalents increased slightly, but not significantly in the small chromosomes (G group). At higher ages, both chromosome groups were not preferentially involved in nondisjunction. Presegregation slightly increased with age and affected more or less all bivalents, whereas the incidence of diploidy significantly decreased. With respect to the rate of meiosis in oocytes from aged females, the resumption was delayed at metaphase I. Our data suggest that failures in the control of oocyte proliferation are involved in nondisjunction rather than the "production-line." Furthermore, a model is proposed to explain nondisjunction of specific bivalents at certain maternal ages.