The growth response of the fetal lung to increased expansion was compared at two gestational ages. In fetal sheep, lung expansion was increased by occluding the trachea for 48 h at either 112-114 days (younger fetuses) or 125-127 days (older fetuses) of gestation (term is approximately 145 days). After 24 h of tracheal occlusion, the volumes of liquid that could be drained from the lungs were increased by 64.7 and 158% above control in younger and older fetuses respectively; the volumes were not increased further after 48 h. In younger fetuses, 48 h of tracheal occlusion increased (p < .05) fetal lung wet weights (21% above control) and protein contents (43% above control) but not DNA contents. In older fetuses, 48 h of tracheal occlusion increased (p < .05) fetal lung wet weights (61% above control), protein contents (41% above control), and DNA contents (22% above control). However, 48 h of tracheal occlusion did not alter total lung hydroxyproline content at either age, resulting in a reduction in the hydroxyproline/protein ratio of the fetal lungs. The results suggest that the lung growth response to tracheal occlusion is greater at 125-127 days of gestation than at 112-114 days of gestation, probably due to a greater accumulation of lung liquid and hence a greater increase in lung expansion, in older fetuses.