Intratracheal instillation of bleomycin in hamsters initiates a series of events that mimic human interstitial pulmonary fibrosis. Because glycosaminoglycans and particularly hyaluronan (hyaluronic acid, HA), may play an important role in the extracellular matrix response to early injury and subsequent fibrosis, this study was undertaken to define the early time course of changes in HA and hyaluronidase. Hamsters were given either 1 unit bleomycin sulfate in 0.2 ml saline or 0.2 ml saline (control), and randomly selected animals from both groups were killed at Days 3, 5, 6, 7, 9, and 17. Glycosaminoglycan fractions prepared from lung tissue of individual animals were analyzed for HA. The maximal HA content was reached 6 days after instillation of bleomycin and was 14.6-fold the normal value. The weight of injured lungs was 2.3-fold the control value. Thus, the increase in HA content was 30-fold. By Day 7 the HA content had dropped sharply. It then declined gradually to approximately double control values at Day 17. The specific activity of lysosomal hyaluronidase was the same in bleomycin-treated lungs and control lungs. Total units of the enzyme were increased in injured lungs, even at the time of maximal HA content, indicating active turnover of HA. The maximal HA content occurs prior to the rise in collagen and elastin biosynthesis. This observation in addition to the magnitude of the increase and its abrupt decline suggest that HA may be an important initiating factor for pathologic changes in lung extracellular matrix components.