A retroviral vector was injected into the heart of developing chicken embryos to determine the best timing for effective gene transduction into the gonads. Transduction efficiency and transgene expression were both highest when the viral solution was injected into embryos at stages 14-15 (Hamburger-Hamilton stages), in which primordial germ cells (PGCs) can be found in the blood circulation. This study shows that PGCs in the blood are important targets for gene transfer into the gonads using retroviral vectors.