In fetal sheep at 113-120 days' gestation, thyroidectomy was performed and tracheal, arterial and venous catheters inserted. Following a recovery period experiments were performed from 120-145 days to measure changes in lung liquid secretion or its absorption in response to I.V. adrenaline infusion or to introduction of dibuteryl cyclic AMP into lung liquid. The results were compared with those previously obtained in non-thyroidectomized fetuses. 2. Plasma levels of thyroid hormones in non-thyroidectomized fetuses confirmed the pattern found by previous workers. In thyroidectomized fetuses the levels of thyroxine (T4), tri-iodothyronine (T3) and reverse T3 (rT3) were very low except in one fetus which showed biochemical evidence of thyroid regeneration towards the end of gestation. 3. In thyroidectomized fetuses the normal response to adrenaline infusion (diminution of reversal of lung liquid secretion) was profoundly suppressed and very little gestational maturation in this response took place, except in the one fetus with evidence of thyroid regeneration in which a normal reabsorptive response developed in late gestation. 4. In thyroidectomized fetuses, the normal response to dibuteryl cyclic AMP was greatly reduced and its increase with gestation which normally parallels that seen during adrenaline infusion did not take place.
