Ontogenic changes of the pulmonary epithelium of the rat, ranging from fetal day 15 to 1 hour after birth (21st day), were observed using light and electron microscopy as well as morphometric analysis on the area occupied by terminal segments of epithelial tubes or by alveolar sacs (ATETAS) in the terminal region of the lung. The development of the lung was classified into four stages. In the pseudoglandular period (fetal days 15 and 16), epithelial tubes lined by columnar epithelium were not yet associated with blood capillaries. The percentages occupied by ATETASs in the terminal region of the lung were 15.3% and 15.7%, respectively, on fetal days 15 and 16. In the precanalicular period (fetal days 17 and 18), blood capillaries began to appose to epithelial tubes lined by cuboidal epithelium. Small osmiophilic lamellar bodies (OLBs) emerged in cuboidal epithelial cells (type II cells) on day 18. The percentages occupied by ATETAS were 24.9% and 25.5%, respectively, on days 17 and 18. In canalicular period (fetal days 19 and 20), sac-like end segments showed progressive thinning of the epithelial linings, and the amount of interstital tissues markedly decreased. The epithelial cells differentiated into squamous, or type I cells, and type II cells containing OLBs. Extrusion of OLBs was recognized on day 20. Most of the capillaries were located close to the epithelial linings. The thickness of the blood-air barrier on day 20 was 10 times that of the adult. The percentages occupied by ATETAS were 35.8% and 38.5%, respectively, on days 19 and 20. In the terminal sac period (in neonate), the wall of terminal sacs showed a thin epithelial lining. Blood capillaries protruded close to the air-way surfaces. The thickness of the blood-air barrier was 3.5 times as thick as that of adult. The percentage occupied by ATETAS was 53.1% of the total terminal region of the lung. When the area occupied by ATETAS at a given day was compared to the area of the adult, the percentage on day 20 was approximately 60% of the adult value, whereas the percentage in neonates was 85%. This suggested that a remarkable transformation took place in ATETAS between day 20 and neonate. In addition to the four periods mentioned above, we also discussed the embryonic and alveolar periods in the development of the rat lung.