Rats 23 days of age were subjected to resection of the upper and middle lobes of the right lung. After 45 days of recovery, their lungs were fixed and the tissue was processed for quantitative light and electron microscopic analysis. Normal and sham-operated animals of identical age served as control animals. At death, the lobectomized rats had normal body weights and lung volumes; both the left lung and the remainder of the right lung participated proportionally in the restoration of the original lung volume. Air space, tissue and capillary volumes, and alveolar and capillary surface areas were as large as those of the control lungs. Tissue composition was slightly altered: the volume proportion of the interstitium was increased at the cost of the epithelium; endothelial volume density and air-blood barrier thicknesses were normal. Nonparenchymal structures had a smaller potential to adapt than the gas-exchanging parenchyma: the volume of conducting airways was smaller than expected for a normal lung. An analogous trend was observed for the larger blood vessels. Based on the recreated dimensions of the gas-exchange apparatus in operated animals, one can assume that the organ fully restores the conditions for adequate gas diffusion.