While a number of workers have described the effects of blast waves upon the lung at both the macroscopic and light microscopic level, studies involving the use of the electron microscope have not been reported. In the experiments reported here the ultrastructural changes seen in lungs from rats exposed to a blast wave impacting on the right side of the chest are described. Considerable damage to the right lower lobe was observed which took the form of tearing of the inter-alveolar septa with capillary rupture and intra-alveolar haemorrhage. Changes to the alveolar epithelium and type II pneumocytes were also noted. Lesions were also identified in the left lung; these included intra-alveolar oedema with a minimal amount of interstitial oedema together with increased pinocytosis and isolated rupture of the alveolar epithelium. 'Ballooning' of the endothelium into the lumen of the capillary was also observed. There was an indication that lesions noted in the left lung at the electron microscopic level may be progressive in the first 24 hours following injury.