The observation that tracheal ligation produces pulmonary hyperplasia even in animals with surgically induced diaphragmatic hernia (DH) has led to rapid application of the technique to human fetuses with DH. The aim of this study was to determine how rapidly fetal lung volume increases after creation of a high-grade tracheal stenosis in fetal sheep with surgically created DH. Twenty-three fetal sheep were prepared with a left thoracotomy at 90 days' gestational. Six had creation of a DH with tracheal stenosis (DHTS) over an 18-gauge cannula, which was then removed. Ten had DH alone, and seven control animals (CT) had a thoracotomy without DH. Thirty days later, vascular and tracheal loop catheters were inserted in all animals and tunneled out the ewes' flank. Between 125 and 140 days' gestation, lung volumes and lung liquid production were measured in awake, unanesthetized animals using a standard double-marker dilution technique. Average lung volumes (in milliliters) were 150.9 ± 13.9 for CT, 29.3 ± 4.4 for DH, and 414.5 ± 88 for DHTS (p < 0.01). Mean lung liquid production varied from 6.00 ± 2.23 mL/h in DH animals before 130 days to 16.69 ± 8.29 mL/h in DHTS animals after 135 days' gestation. DH animals had lower lung liquid production (8.51 ± 1.4 mL/h) than CT (12.4 ± 0.8 mL/h) or DHTS animals (12.4 ± 2.2 mL/h) (P < .01). The rate constant γ (h-1) for lung liquid production was significantly higher in DH animals than in either CT or DHTS animals (P < .01). Tracheal stenosis in this model causes rapid lung growth before 130 days' gestation. The authors speculate that short periods of incomplete stenosis might reverse the pulmonary hypoplasia associated with DH. To achieve this goal, the timing and duration of treatment and the optimal degree of stenosis must be defined.