We have determined whether changes in lung hyaluronan content affect extravascular water in lungs of unanesthetized rabbits. Three groups of experiments were performed. In group 1 (n = 12), no infusions were given; in group 2, nine pairs of rabbits received either intravenous hyaluronidase (750 U.kg-1.min-1) or an equivalent volume of saline; in group 3, nine pairs of rabbits received either hyaluronidase or saline, followed by intravenous saline infusion amounting to 24% of body weight. At the end of each experiment, one lung was analyzed for extravascular lung water by the wet-dry method. Except for group 3, in all animals the other lung was analyzed for hyaluronan content by a method that involved hydrolyzing lung hyaluronan with fungal hyaluronidase to release reducing N-acetyl glucosamine groups, which were quantified. In group 1, lung hyaluronan, which varied from 50 to 159 micrograms/g dry wt (mean 106 +/- 35 micrograms/g dry wt), significantly correlated with variation in extravascular lung water (mean 4.2 +/- 0.3 g/g dry wt). In group 2 rabbits given hyaluronidase, lung hyaluronan was 40% lower and extravascular lung water was 14.6% lower than in paired controls (P less than 0.01). In group 3, volume expansion did not affect lung water, except after hyaluronidase when lung water was 47% higher than paired controls. We conclude that in the lung the content of hyaluronan is one of the determinants of extravascular water content.