1. Following thyroidectomy at 106-118 days fetal sheep were infused continuously with triiodothyronine (T3) from 110, 118, 125 or 131 days (n = 12) or with thyroxine (T4) from 118 days (n = 4) until the fetuses were delivered. Lung liquid secretion or absorption rates, heart rate, blood pressure and arterial blood gases were measured before and during 45 min periods of fetal infusions of adrenaline (n = 60) at 3-8 day intervals. The effects of T3 or T4 replacement on the response to adrenaline were compared with data previously obtained in groups of euthyroid (control) and thyroidectomized (Tx) fetuses. 2. Fetuses infused with T4 (50 micrograms/day) following thyroidectomy had plasma T4 and T3 concentrations in the normal fetal range. Fetal plasma T3 levels in fetuses infused with T3 (60 micrograms/day) were at or above the high end of the normal range for full-term fetuses. Those receiving 120 micrograms of T3 per day had levels equivalent to those normally seen in the postnatal T3 surge. 3. Normal maturation in the lung of the reabsorptive response of fetal lung liquid to adrenaline was seen in the fetuses infused with T3 or T4 from 118 days. A marginal advance in maturation was seen in fetuses infused with T3 from 110 days and a delay in maturation in those infused with T3 from 125 and 131 days.