We have studied the effects of soluble factors obtained from rat lungs with experimentally-induced pulmonary fibrosis of 2 months duration on the in vitro rates of biosynthesis and degradation of collagen in normal rat lung preparations. Factors soluble in phosphate-buffered saline were prepared from the minced lungs of normal controls and of silicotic animals. The in vitro rate of collagen biosynthesis of normal rat lung explants was measured as the rate of incorporation of radioactive proline into total and collagenous protein. The in vitro rate of collagen degradation in normal rat lung homogenates was measured as the rate of release of hydroxyproline-containing materials of less than 100,000 daltons to the supernatant. Our results suggest that in this experimental model of pulmonary fibrosis there are soluble factors that stimulate both collagen biosynthesis and collagen degradation in in vitro preparations of normal rat lung.