Quantitative changes of lung tissue components (air spaces lined by PAS-positive and PAS-negative epithelium, blood vessels and interstitium) were investigated in developing rats from fetal day 18 through neonatal day 1. The volume of the left lung increased significantly from fetal day 18 through neonatal day 1. The percentage and volume of the air spaces increased strikingly between fetal days 20 and 21. However, the percentage of the air spaces lined by PAS-positive epithelium decreased significantly from fetal days 20 to 21, and that of the spaces lined by PAS-negative epithelium increased between the two days. The proliferating cell nuclear antigen (PCNA)-positive cells were rich in the interstitium and epithelium of the air spaces on fetal days 18 and 19. The percentage of the interstitium decreased significantly from fetal day 18 through neonatal day 1, showing remarkable decrease between fetal days 20 and 21. From fetal day 20 onward, the PCNA-positive cells decreased in number and located in the epithelium of the conducting air ways and interstitium. Based upon these findings, the present study suggests that the period from fetal days 20 to 21 is a critical time for the development of fetal lung: the period before fetal day 20 is that for proliferation and the period after fetal day 21, functional differentiation of the lung.