SummarySaline or triiodothyronine (T3) (50‐1000 &mgr;g) was injected into the amniotic sac of 17 pregnant ewes under general anesthesia at 130 days gestation. Forty‐eight h later, the lambs were delivered by hysterotomy. Cord plasma T3and cortisol and amniotic fluid T3were assayed, and the maturity of the fetal lung was assessed in terms of its pressure‐volume response and its surfactant (lamellar body phospholipid) content. With the highest dose of T3, cord plasma T3and cortisol were raised, and lung maturity was enhanced compared with saline‐treated controls; the pressure‐volume curve showed increased hysteresis on inflation and deflation, and the lung retained air on return to zero pressure. There was also an apparent, but not statistically significant, increase in the lamellar body phospholipid content of the lung. Irrespective of treatment, lungs which were more mature, in terms of their pressure‐volume characteristics, tended to contain a higher proportion of lamellar body phospholipid relative to total phospholipid.AbbreviationsT4, thyroxineT3, triiodothyronine