The transplanted immature rat lung has been shown capable of fulfilling its growth potential after left lung transplantation (LLTx) and concomitant right cardiac lobe resection (RCLR). However, unexpected findings in these studies included an abnormal rise in lung volume in the contralateral lung due to alveolar multiplication, and significant dilatation of the bronchi in the transplanted left lung. In the present study, the influence of surgical factors that could have contributed to these changes, i.e. RCLR, denervation, and anastomotic stenosis, were studied individually. Immature Lewis rats aged 4 and 6 weeks were used and (a) RCLR, (b) RCLR and right hilar stripping, and (c) RCLR and left pulmonary artery (PA) banding were performed in groups 1 (n = 12), 2 (n = 5) and 3 (n = 6), respectively. Animals were killed after 6 months and the lungs studied using quantitative morphometric techniques. In groups 1 and 2, right lungs did not show any significant increase in volume. Alveolar number and airway diameter in both lungs in all groups were not significantly different from controls. In group 3 both right and left lungs presented an increase in alveolar size (p<0.02 on the right and /><0.01 on the left). Changes seen after LLTx in the rat, such as alveolar multiplication, cannot be explained by compensatory growth (RCLR), denervation (RCLR and hilar stripping), or arterial stenosis at the anastomotic site (RCLR and left PA banding), but must be regarded as a consequence of transplantation per se. A role for neuropeptides in lung growth following transplantation is suggested. [Eur J Cardio-thorac Surg (1991) 5:218-222]. © 1991 Springer-Verlag.