Previously, this laboratory has shown that intratracheally administered hyaluronic acid (HA) significantly reduces air-space enlargement in a hamster model of emphysema induced with pancreatic elastase. Whereas HA was given immediately following elastase in those initial studies, the current investigation determined the effect of instilling HA up to 2 h before or after intratracheal administration of elastase to hamsters. Both 1 and 2 mg HA, given 2 h before pancreatic elastase, significantly decreased (p < .05) air-space enlargement compared to controls (as measured by the mean linear intercept). Instillment of 2 mg HA, 1 h after pancreatic elastase, had a similar effect (p < .05). In contrast, 1 mg HA, given 1 or 2 h after pancreatic elastase, did not significantly affect the mean linear intercept. Against human neutrophil elastase, HA exhibited the same protective effect. While neutrophil elastase induced less air-space enlargement than pancreatic elastase, both 1 and 4 mg of HA, given 2 h prior to the enzyme, still produced a significant reduction (p < .05) in the mean linear intercept. HA exerted this effect despite the fact that it initiates a transient influx of neutrophils into the lung. Since HA does not slow the clearance of intratracheally instilled [14C] albumin from the lung, its mechanism of action may not involve physical interference with the movement of elastase through the lung, but may instead depend on interaction with elastic fibers. Evidence for an association between these two matrix constituents was provided by studies using fluorescein-labeled HA. Overall, these results further suggest that HA may be useful in preventing lung injury by elastases.