Lung lavage fibronectin is known to be elevated after experimental lung injury; however, changes in lung tissue fibronectin content probably more directly reflect tissue injury and repair. We injured rat lungs with a single injection of paraquat and serially measured the fibronectin content of both lung lavage fluid and lung tissue. Changes in lung ultrastructural morphology and collagen content were also assessed. Although both lavage and tissue content of fibronectin increased, the temporal patterns of increase were very different. Lavage fibronectin peaked at 1 day post-injury, was normal by 7 days, and paralleled changes in lavage albumin. Tissue fibronectin was normal at 1 day post-injury, peaked at 4 days and was still high at 7 and 14 days. The period of increased tissue fibronectin coincided with electron microscopic evidence of cell injury and repair at 7 days. By 14 days post-injury the lungs appeared virtually normal. Lung collagen content remained normal throughout the period of injury and repair. We conclude that increases in lung lavage fibronectin do not necessarily correlate with changes in lung tissue fibronectin and that the latter may more accurately reflect lung tissue repair. In addition, even large increases in both measurements do not necessarily predict the development of post-injury fibrosis.