RT3(3,3',5'-triiodothyronine) levels in amniotic fluid and T4(thyroxine), T3(triiodothyronine), rT3 and TSH(thyroid-stimulating hormone) levels in maternal and cord serum were determined simultaneously by RIA. We also determined the activities of the monodeiodination of thyroxine to rT3 in placentas. Amniotic fluid rT3 and cord serum rT3 levels decreased, but T4, T3 and TSH levels increased with advancing gestational age. The activities of the monodeiodination in placentas decreased rapidly from midgestation, preterm to term. In maternal hyperthyroidism, amniotic fluid rT3 levels were markedly elevated. Moreover, there were significant positive correlations between amniotic fluid rT3 and maternal serum rT3 (r = 0.756, p less than 0.001, n = 26) and T4(r = 0.509, p less than 0.01, n = 26) in the normal 3rd trimester. We found significant correlations between amniotic fluid rT3 and fetal thyroid function as well as the activity of the monodeiodination in placenta after 17 weeks' gestation. But we couldn't find any such correlations in the 3rd trimester. These data suggest that the amniotic fluid rT3 in the 3rd trimester was affected by maternal thyroid function as well as fetal thyroid function and the activity of the monodeiodination in placenta.