Syngeneic (Lewis-to-Lewis) and allogeneic (Brown Norway-to-Lewis) unilateral left lung transplants were performed between immature rats aged 6 wk (donors) and adult animals aged 4 mo (recipients). Transplanted animals were killed either 2 wk or 6 mo after operation. Right and left lungs were analyzed separately by quantitative light microscopic techniques, and findings were compared with control animals matched for age and strain. The immature transplanted left lung continued to grow to achieve its predicted adult number of alveoli. Six months after transplantation the alveoli and airways were larger than controls in allogeneic animals (P < 0.001) but were of normal size in syngeneic animals. After both syngeneic and allogeneic transplantation the recipient contralateral mature native right lung showed an increase in volume after 6 mo (P < 0.001), abnormal in a mature lung, due to an increase in size (P < 0.01 in allogeneic, P < 0.05 in syngeneic) and alveolar number (P < 0.02 in both). This study indicates that in rats, transplanted immature lungs can fulfill their growth potential even when transplanted into mature recipients.