Female rats are masculinized in utero by male littermates sharing the same uterine horn. Increased anogenital distances in neonatal females and mounting behavior in adult females are related to the presence of males on the caudal side of the females in the uterine horn. Contrary to current beliefs, interamniotic diffusion may not be responsible for the exchange of masculinizing agents among fetuses. Since uterine blood flow in the rat is from the direction of the cervix toward the ovary, masculinizing hormones secreted by fetal males may be carried via the uterine vasculature to female littermates located further downstream.