Embryos of the Rouen duck were treated on day 11 to 18 with a single injection of estradiol benzoate in the air chamber. After hatching, skin grafts were exchanged between male ducklings that had been treated with estrogen in ovo and control males. The results showed that the skin of male embryos can be permanently feminized to the same extent as untreated female embryos simply by exposing embryonic male skin to female sex hormones during embryogenesis.