We observed for differential effects of maternal treatment with dexamethasone, triiodothyronine (T3), or both in late gestation on the growth of fetal rat lungs during oligohydramnios-induced pulmonary hypoplasia (PH). Timed-pregnant mothers were randomly selected into four treatment groups: controls (no hormone treatment); dexamethasone only; T3 only; and both dexamethasone and T3. Each underwent amniocentesis of one uterine horn on day 15 of gestation. Untouched littermate on the opposite horn served as internal control. On days 19 and 20, treated mothers were given dexamethasone (0.2 mg/kg) or T3 (7 mg/kg intramuscularly), or both and were delivered on day 21 (term) by hysterotomy. Amniocentesis resulted in PH, defined as decreased wet lung weight to body weight ratio and lung DNA contents, 83% and 90% of control, respectively (P < 0.05). Body weight and lung weight decreased with hormone treatment for both with and without amniocentesis. Although hormone treatment resulted in smaller lungs, there was no significant difference in lung weight to body weight ratio for either group with or without amniocentesis. This suggests that hormone treatment resulted in proportionate growth retardation. All hormone treatments decreased the total lung DNA content during amniocentesis (P < 0.05). Growth suppression of fetal lung associated with maternal hormone treatment is superimposed on the pulmonary hypoplasia induced by oligohydramnios.