The concentration of gonadotropin-releasing hormone (GnRH) in the placenta, maternal plasma and fetal hypothalamus was measured in rats during days 13–21 of gestation. A substantial amount of GnRH was detectable in the extract of placenta and maternal plasma samples collected between days 13 and 21 and in the extract of fetal hypothalamus collected between days 19 and 21 of pregnancy. The GnRH level in the placenta and fetal hypothalamus, but not in plasma, fluctuated significantly during pregnancy. The level of GnRH gradually decreased in the placenta but increased in the fetal hypothalamus as the pregnancy proceeded. After gel filtration of placental extract, GnRH eluted in the same position as synthetic GnRH and hypothalamic GnRH. Dilution of placental extracts produced a displacement curve parallel to that of GnRH and hypothalamic extracts. These results indicate that GnRH is present in the placenta and that placental GnRH may have a role in the maintenance of pregnancy in rat