The histopathology of rat lung has been studied after an acute exposure to perfluoroisobutylene (PFIB) at a concentration of 638 mg/m3 (78 p.p.m.) for 1.5 min giving a Ct = 957 mg min/m3 for the first 24 h following exposure. Within 5 min of exposure changes to the bronchioles and peribronchial alveoli were observed which took the form of alterations to cilial structure, increased pinocytosis and electron lucency, with occasional vesicle formation of type I alveolar epithelial cells. Intercellular leakage with minimal fluid accumulation in the alveolar spaces was also seen. The very rapid action of PFIB strongly suggests a direct action by the compound. There then followed the gradual development of pulmonary oedema which was visible histologically 2-3 h post exposure with deaths occurring from 7 h onwards. Animals sacrificed at 24 h post exposure showed evidence of widespread pulmonary oedema and alveolar interstitial infiltration by lympho-mononuclear cells and macrophages.