Pulmonary structure and function were quantitatively investigated over the lifespan of the Fischer 344 rat by morphometric and physiologic techniques. Male animals 1 week, 6 weeks, 5 months, 14 months, and 26 months of age and female animals 5 months, 14 months, and 26 months of age were studied. All alveolar tissue compartments demonstrated significant increases in volume, surface area, and cell number during the first 5 months of life. From 5 to 26 months of age, remodelling in the epithelial and interstitial compartments continued to take place while the endothelial compartment remained relatively unchanged. In the epithelial compartment the ratio of type II cells to type I cells lining the alveolar surface decreased as age increased. In the interstitial compartment the volume of the noncellular components of the interstitium increased by 39% in males and by 89% in females from 5 to 26 months of age. Physiologic measurements of lung volumes in males at 6 weeks, 14 months, and 26 months demonstrated progressive increases in vital capacity (VC) and total lung capacity (TLC). Morphometric pulmonary-diffusion capacity (DLO2) increased in males from 1 week to 5 months of age and remained relatively unchanged from 5 to 26 months of age in both sexes.