Concentrations of immunoreactive (ir-) inhibin in circulation, amniotic fluid and the ovary of embryos and newly-hatched ducks from Day 21 of incubation to one day of age were determined. The antiserum used was against bovine 31-kDa inhibin and was validated for RIA of inhibin using samples from female embryos. Plasma concentrations of FSH and LH were also determined by chicken RIA systems. Plasma concentrations of ir-inhibin were maintained at constant levels from day 21 to day 25, abruptly increased on day 26. After the increase, plasma concentrations of ir-inhibin quickly declined from day 27 to the day of hatch (day 28). Plasma concentrations of FSH were high on day 21, followed by an abrupt decline on day 22, gradually increased until day 24, and then decreased again on day 25. The maximal rise of plasma FSH was observed on day 27, and then rapidly decreased until the day after the hatch. No inverse relationship was observed between plasma concnetrations of ir-inhibin and FSH. Amniotic fluid concentrations of ir-inhibin were relatively low and remained constant between Day 21 and Day 25. Embryonic ovaries contained very low amounts of ir-inhibin. Ovarian inhibin levels were stabilized in early time points, temporally decreased on day 25, and abruptly increased on day 26. These results suggest that inhibin does not involved in the regulation of FSH secretion in the female duck embryo and the ovary of duck embryo is not the main source of circulating ir-inhibin.