Cyclophosphamide is a widely used anticancer drug which damages lung tissue and elicits a progressive fibrotic lesion in mice. The early time course of collagen accumulation and the changes in collagen synthesis which may accompany this lung damage and fibrosis have not been reported. The total lung concentration of hydroxyproline, an index of collagen content and the extent of pulmonary fibrosis, was not significantly increased in mice until 12 days after treatment with 200 mg/kg cyclophosphamide. The change in total lung hydroxyproline content was preceded, on Day 6, by an increase in the rate of acid-insoluble hydroxyproline synthesis. This rate remained elevated to Day 21, but was no longer significantly increased by Day 28. The percentage of total protein synthesis devoted to the production of collagen was significantly increased 9 and 12 days after cyclophosphamide, compared to saline-treated controls. The rate of pulmonary non-collagen protein synthesis was significantly decreased 3 days after cyclophosphamide, and increased 6 and 15 days after this treatment. These data indicate that cyclophosphamide-induced increases in pulmonary collagen synthesis precede the accumulation of this protein. Increased collagen synthesis may occur in the absence of changes in overall protein synthesis although cyclophosphamide also alters non-collagen protein synthesis.