Sex of day-12 rat embryos was determined using Barr body counts made on spreads of amniotic membranes examined histologically. Embryonic weight, protein content and rate of thymidine incorporation were compared in male and female embryos. Male embryos were found to be heavier and accordingly to contain more protein on absolute but not on per unit weight basis. The rate of thymidine incorporation did not differ in the two sexes. Since gonadogenesis in day-12 rat embryos is rudimentary, with gonadal differentiation of sex not yet apparent, the increased weight suggests that sex-linked genes exist which influence body growth prior to gonadal endocrine activity.