The rapidity with which lung growth is initiated and completed after pneumonectomy was examined in young rats (4 wk of age; 82 g). After left pneumonectomy, the remaining lobes of the right lung grew to equal the weight of both lungs of control animals by day 7 and within 14 days increased from 366 to 968 mg. The tissue concentrations of RNA, DNA phosphate, collagen, and noncollagen proteins did not increase during the growth response. In contrast, total amounts of these constituents increased significantly in the remaining lung of pneumonectomized animals during the 1st postoperative wk and approached levels found in both lungs of sham-operated and unoperated controls by the end of the 2nd wk after pneumonectomy. Although cell size increased in control lungs during the experimental period, there was little evidence of additional cellular hypertrophy associated with compensatory lung growth. The character of the response to pneumonectomy in these rats was similar to that observed previously in older animals (320 g). Thus in spite of the higher basal rate of lung growth in the younger rats, the pattern and rapidity of compensation after pneumonectomy was similar in both age groups.