Lung growth was accompanied by alveolar multiplication in rats from 4 to 10 wk of age; the multiplication then ceased until 14 wk of age, and in this latter interval the alveolar walls appeared to lengthen. The relationship between size of surface alveoli and size of internal alveoli of the lower lobe changed with age, and the relationship was not altered by pneumonectomy. The surface alveoli were smaller than internal alveoli at 4 wk of age, the same size at 6 and 10 wk of age, and larger at 14 wk of age. Four-week-old rats responded to pneumonectomy with an increase in size of the contralateral lung and an increase in the number of alveoli. Direct alveolar counts did not show a significant increase in number of alveoli after pneumonectomy in 8- and 12-wk-old rats. the alveolar surface area incresed almost directly with the increase in gas-exchanging lung volume in 8-wk-old rats, suggesting that alveolar multiplication might have occurred. In 12-wk-old rats, however, the alveolar surface area increased to the 0.71 power of the increase in gas-exchanging lung volume, suggesting that alveolar enlargement, rather than alveolar multiplication, had occurred. Taken in conjunction with the data for growth in normal and sham-operated animals, these results suggest that compensatory alveolar multiplication is part of the adaptive response to pneumonectomy when this operation is performed at a time at which alveolar multiplication normally occurs. When pneumonectomy is performed after alveolar multiplication has ceased, the adaptive response is primarily one of air-space enlargement. Lung volume, weight, surface area, and protein responses to pneumonectomy were smaller at 12 wek of age than at 4 and 8 wk of age.