Gonadotropin-releasing hormone was localized immunohistochemically during implantation (gestational days 6 to 14) in the New Zealand White rabbit. During early implantation (days 7 to 9), intense gonadotropin-releasing hormone immunostaining was localized predominantly to the cytoplasm of the nonknob cytotrophoblast with light to moderate staining in the cytoplasm of the syncytiotrophoblast (knob). In later gestation, light to moderate staining of the cytoplasm of the trophoblast at the true placental site was detected. No appreciable change in staining was noted after day 10. Fetal membranes, identified after day 10, showed intense and unchanging immunostaining for gonadotropin-releasing hormone. Obplacental giant cells showed light to moderate nuclear and cytoplasmic gonadotropin-releasing hormone immunostaining. Light to moderate gonadotropin-releasing hormone immunostaining was also noted in the cytoplasm of uterine epithelium and glands. We conclude that immunoreactive gonadotropin-releasing hormone is present in the cytotrophoblast at the time of the earliest embryo-uterine interactions and may play a significant role in implantation and embryo survival.