We examined the growth and development of XO embryos at mid-gestation (day 10) by assessing crown-rump length, head length, somite number, and morphological score (Van Maele-Fabry et al., 1990), using an XO mouse colony established from the Jcl/ICR strain. Contrary to expectation, there were no differences between the XO and XX embryos in any parameter measured. Also, no particular region was specifically retarded in XO embryos during development. We therefore concluded that the growth and development of XO embryos were not retarded when compared with their XX littermates at day 10 of gestation.