1. Mouse and rat lungs punctured by cold sterile needles show local atelectasis with a transient and reversible swelling of the alveolar lining cells. No evidence of the injury can be seen, by naked-eye or microscopically, a week later. 2. Burns of lung produced by heated needles are organised from a boundary or demarcation zone of lung tissue and converted to functioning lung by the ingrowth of bronchial branches from pre-existing bronchi. 3. Excision of wedges of lung tissue in cats is followed by local scarring and the ultimate reformation of lung tissue in the scar. This process occurs through bronchial proliferation together with alveolar formation in the margins of the scar, processes which depend primarily on the active motility of the lung
