The meiotic segregation pattern of the X chromosome in XO mice was determined by counting second meiotic metaphase chromosomes. A total of 584 oocytes was collected from superovulated but unmated XO and XX mice. Second meiotic metaphase (MII) counts indicated that the X chromosome in XO mice did not segregate randomly but remained preferentially in the oocyte. Increased maternal age had no effect upon this segregation pattern. Selection against XO and YO preimplantation embryos was evaluated by observing their relative frequencies during first and second cleavage divisions. A combined total of 1650 preimplantation eggs was collected from superovulated and mated XO and XX female mice. Eggs were obtained during metaphase of first (CI) or second cleavage (CII) divisions by injecting mated females with colchicine at 28 or 49 hr, respectively, after human chorionic gonadotrophin (HCG) administration. Prior to analysis, chromosomes were banded by trypsin-Giemsa, which facilitated specific chromosome identification, including the X and Y. The proportion of XX, XY, XO, and YO embryos observed at CI from XO females was not significantly different from expected values based on MII counts. At CII, a significant difference was observed from the expected proportion of embryo classes resulting from a reduction in XO and YO embryos. The results provide cytogenetic evidence emphasizing that, in order for development to continue, at least one functional X chromosome is necessary as early as the two-cell stage. Triploid one- and two-cell embryos were observed more frequently in embryos from XO than XX females. The sex ratio was not significantly different from 1:1 during either cleavage stage regardless of the type of female from which the embryos were derived.