Following preliminary results in animal studies demonstrating that thyroxine (T4) injected directly into the fetus or the amniotic sac is capable of enhancing fetal lung maturity, a trial of thyroxine treatment in mean was carried out. Two hundred micrograms of T4 were injected intra-amniotically in eight women with high-risk pregnancies in which premature delivery was inevitable or indicated. Fetal lung maturity was evaluated by a recently developed, accurate, and rapid method using the microviscosity value epsilon (this being the linear expression of the polarization value P) of the lipid assembly of the amniotic fluid as a parameter of the surfactant content. epsilon Values, which were very high before T4 injection, dropped steeply to below the value of 4.344, indicating fetal lung maturity. Eight newborn infants delivered, five by caesarean section, soon after the T4 injection did not develop respiratory distress syndrome, although they were born prematurely. Intra-amniotic injection of T4 appears to be of therapeutic value in enhancing fetal lung maturity.