Observations by light microscopic morphometry and scanning electron microscopy were performed on the lungs from 86 specific pathogen-free Fischer 344 male rats between 1 day and 32 months of age. The distribution curve of the mean chord length of the gas exchanging area appeared as a single peak (approximately 70 microns) at day 1, and two peaks (approximately 50 and 90 microns) were seen at day 7 when the first alveoli appeared. At 3 months of age, the distribution curve peaks began to decrease gradually, becoming more flattened with a wide base to a maximum 200 microns. Between 27 and 32 months of age, ductectasia occurred and the alveolar surface appeared more irregular and rough, but no destruction of the alveolar wall was observed. From these observations, it was concluded that the first alveoli appear by 7 days of age in male Fischer 344 rats, that the alveolar size gradually increases after 3 months of age, and that ductectasia appears after 27 months of age. These changes might reflect changes in the matrix of the alveolar walls due to nutritional deterioration in old age, concomitant with cellular atrophy of this zone.