Nuclear sizes were measured in 1000 cells each of four human fibroblast cultures, two of which were of normal XX-, and two of normal XY-karyotype. In the XX-cultures, the nuclei containing a Barr body were significantly smaller than those lacking a Barr body. The difference in size was of the order of 6%. It is suggested that the smaller nuclear size is partly a direct effect of the condensation of one X-chromosome and partly an indirect effect, Barr bodies being present mainly at those stages of the mitotic cycle when the nucleus is not at its maximum size.