We reported previously on the beneficial influence of intra-amniotic T4 injection in accelerating human foetal lung maturity. Twenty-four h after T4 (250 μg) intra-amniotic injection, the mean cord serum T4 concentration in 13 premature newborns was 17±2.1 μg/100 ml, while the T4 concentration in a matched group was only 11.6±0.9 μg/100 ml: in 5 of the infants who received intra-amniotic thyroxine T4 was measured 12 h post-partum and rose from 16.1±1.8 to 24.3±2.6 μg/100 ml. No change in maternal T4 concentration was observed. The mean cord serum T4 concentration in premature newborns delivered more than 3 days following T4 intra-amniotic injection, however, was not significantly different from that of the controls. These data suggest that T4 (250 μg) injected intra-amniotically is absorbed by the premature human foetus. Furthermore, the hyperthyroxinaemia is only transient, and does not prevent the post-natal T4 surge.