Experiments were conducted to determine if sex differences in body weight of chickens exist prior to hatching and to determine if growth of twin embryos is affected by the sex of their siblings. The mean wet BW of male embryos was significantly greater than that of females at 11, 13, and 18 days of incubation, whether expressed on an absolute basis or as a percentage of egg weight. The mean dry weight of male embryos was not different from that of females at 11 days of incubation. Male dry weights were not different from female weights at 13 days of incubation in one experiment (P = .540) but approached significance (P = .078) at this age in another experiment. The mean dry weight of males was significantly greater than that of females at 18 days of incubation. Vascular anastomoses between twin embryos in double yolk eggs were demonstrated by the exchange of 125I between the twins. Fifteen-day-old twin embryos in double-yolked eggs were significantly lighter than single embryos developing in double-yolked eggs. Female embryos twinned with females were slightly but not significantly lighter than males twinned with males. Mixed sex twins tended to be lighter than unisex twins. Females twinned with males were significantly lighter than males twinned with males. The data do not support the concept that a male humoral factor is responsible for more rapid growth of male embryos.