Increased vascular collagen content is a major feature of pulmonary vascular remodeling. The functional role of excess collagen in decreasing pulmonary vascular compliance has not been established. We determined whether there was a correlation between hydroxyproline content of rat pulmonary artery segments and elastance (EPA) of the pulmonary artery bed during development of hypoxic pulmonary hypertension (10% O2, 10 d) and normoxic recovery. EPA was measured by air-filled pressure-volume curves. After 10 d of hypoxia, hydroxyproline content increased approximately 2-fold in large segments (1,200-250 microns in diameter) but not significantly in small segments (> 250 microns). Elastance increased from 87 +/- 6 (SEM) to 145 +/- 8 mm Hg/ml (p < 0.05) within 5 d of hypoxia and returned to control value 3 wk after recovery. There was a correlation between collagen content and EPA in large segments during development of hypertension; no correlation was found during recovery from hypoxia. The ratio of hydroxyproline to total protein was unchanged in large segments after recovery from hypoxia but was increased in small segments after recovery. We conclude that increased collagen in large pulmonary arteries directly influences EPA during the development of hypoxic pulmonary hypertension.