To assess the genetic component of the maternal age effect, the frequencies of aneuploid embryos derived from several inbred mouse strains were examined as a function of maternal age. Some of the strains were chosen to evaluate possible etiologic agents: CBA for premature reproductive aging and increased univalent frequency, and NZB J and A J for predisposition toward autoimmunity. The C3H HeJ and C57BL 6J strains also were examined. Although there was considerable variation between strains, an increased frequency of aneuploid fetuses was observed in all strains as a function of maternal aging. The CBA strain revealed the greatest increase in frequency of aneuploid fetuses at a relatively early maternal age. The effect of autoimmunity was assessed by analyzing the frequency of aneuploid embryos derived from thymectomized A J females. No significant difference in aneuploidy was found between thymectomized and control nonthymectomized animals.