To study the influence of blood flow on postpneumonectomy lung growth, we banded the left caudal lobe pulmonary artery of eight ferrets in such a way that blood flow to the caudal lobe did not increase when the right lung was excised 1 wk later. The fraction of the cardiac output received by the right lung before pneumonectomy was therefore directed entirely to the left cranial lobe. Three weeks after pneumonectomy the weight, volume, and protein and DNA contents of the two lobes of the left lung were measured and compared with those of five unoperated animals and eight animals after right pneumonectomy alone. Although its perfusion did not increase after pneumonectomy, the left caudal lobe of banded animals participated in compensatory growth, increasing in weight and protein and DNA contents. Although the cranial lobe of banded animals received 25% more of the cardiac output than the same lobe in pneumonectomized animals, cranial lobe volume and protein and DNA contents in the two groups were similar. Caudal lobes were smaller in banded than in simple pneumonectomized animals and tended to contain less protein, whereas the cranial lobes tended to be heavier. We conclude that increased pulmonary perfusion is not necessary for compensatory lung growth in adult ferrets, but it may modify this response.