This study examined whether the improvement in lung function after prenatal hormone exposure coincided with changes in lung morphometry or in collagen and elastin content. Fetal lambs received a single intramuscular injection of betamethasone (0.5 mg/kg) plus L-thyroxine (T 4)(15 µg/kg) or vehicle control 48 h before delivery at 121, 128, or 135 d gestational age (d 121, d 128, d 135, term= 150 d). T 4 was administered in conjunction with betamethasone in an attempt to enhance the maturational response. The right-upper lobes were instillation fixed at 30 cm H 2 O by Karnovsky's fixative after a 40-min period of mechanical ventilation. A number of significant changes occurred between d 121 and d 135 in control animals: alveolar airspace volume increased by 270%; despite a 40% reduction in alveolar septal thickness, alveolar septal volume did not change appreciably, suggesting a" redistribution" of septal tissue into the formation of secondary alveolar septa, which doubled in number; and both parenchymal collagen and elastin volume increased significantly, whereas pleural collagen and elastin volume did not change. In contrast to the changes seen in control animals, exposure to betamethasone plus T 4 led to alveolar septal thinning at each gestational age without an associated increase in secondary septal number, a 40% decrease in alveolar septal volume, and a proportionate reduction in parenchymal elastin at d 121. Although attenuation of alveolar septa coincides with redistribution of septal tissue into the formation of secondary septa during normal maturation, exposure to betamethasone plus T 4 promotes thinning of alveolar septa in the absence of secondary septal formation, which results in a loss of alveolar septal tissue.