Congenital diaphragmatic hernia (CDH) continues to carry high morbidity and mortality. A number of treatment modalities including extracorporeal membrane oxygenation and in utero repair have improved the mortality rate only minimally. With this condition, there is often insufficient lung mass at birth and persistent pulmonary hypertension postnatally. Experimental fetal tracheal ligation has been shown to increase lung growth in utero and to reduce the hernial contents in CDH. The purpose of this study was to determine the effect of reversible fetal tracheal occlusion on tracheal pressure and lung development. Nine fetal sheep were divided into two groups. Group 1 had intratracheal balloons placed, and the balloons were left inflated for 21 to 28 days. Group 2 consisted of littermates that served as controls. They either had uninflated balloons placed or were left unoperated. Tracheal pressure measurements were recorded periodically, and the amniotic fluid pressure served as a reference. The animals were killed near term, and the lungs, heart, and liver were weighed and corrected for body weight. Standard morphometry was used to compare the lungs further, and the lung DNA and protein content were measured. Tracheal damage from the balloon catheter also was assessed. The tracheal pressure was 3.85 (+/- .49 SE) mm Hg in experimental animals, and it averaged -0.27 (+/- .27 SE) mm Hg in controls (P < .0001). Tracheal occlusion increased lung weight and volume by two to three times (P < .0001 and P < .0006, respectively) while heart and liver weights remained similar to those of controls.(ABSTRACT TRUNCATED AT 250 WORDS)