Syngeneic (Lewis to Lewis) and allogeneic (Brown Norway to Lewis) unilateral left lung transplants were performed on immature rats at 6 weeks of age at a time when alveoli are still multiplying after birth. Left lung denervation without transplantation was performed in a further group of rats (Lewis by stripping the hilum, at 4 and 6 weeks of age. Animals were killed at either 2 weeks or 6 months after operation. Right and left lungs were analyzed separately by light microscopic quantitative techniques and findings were compared with findings from control animals matched for age and strain. The transplanted left lung in both syngeneic and allogeneic animals continued to grow to a normal size by formation of new alveoli, despite the presence of low-grade rejection activity in the immunosuppressed allogeneic group. The airways showed an increase in diameter for age at the hilum and periphery (p < 0.01 and < 0.001, respectively). The volume of the contralateral right lung was greater than normal because of an increase in number (p < 0.01) and size of alveoli for age. Denervation alone was associated with normal growth of both lungs. Thus it appears that, in rats, the transplanted immature lung can fulfill its growth potential.