SUMMARYLifetime reproductive performance was studied in 50 sib-pairs of female mice, one member of each pair chromosomally XX and the other XO. Twenty-five pairs were irradiated with 25 rad X-rays at 10 days of age and 25 were unirradiated. In both the irradiated and control series the XO mice had a significantly shorter reproductive life than the XX ones, but unirradiated XO bred longer than irradiated XX. The median age of unirradiated XO at birth of last litter was 280 days and they had 6·5 ± 0·80 litters, whereas for XX the figures were 420days and 12·6 ± 0·74. The mean litter-size of XO mice was only about 55% and their lifetime productivity 34% of that of their XX sibs. Similarly, the lifetime productivity of irradiated XX females was only 31% of that of unirradiated XX. Histological studies showed that in the unirradiated XO mice reproduction ended through shortage of oocytes, and the resulting secondary ovarian changes were similar to those in irradiated mice. Thus, the differences between human and mouse XO types are to some extent reconciled. Both become sterile through death of oocytes, in humans before puberty and in mice after. This difference may be connected with different times from oogenesis to puberty in the two species.