Male Sprague-Dawley rats were subjected to a single, intratracheal instillation of 30 mg Min-U-Sil silica in sterile saline and were sacrificed 3, 7, or 14 days following instillation. Control animals were instilled with sterile saline only. Silica instillation produced an inflammatory reaction followed by histological changes characteristic of lung fibrosis. Thickened alveolar septa associated with inflammatory cells transforming into large multifocal fibrotic nodules were detected in silica-exposed animals. Increased numbers of bronchoalveolar cells (principally macrophages), elevated levels of protein (principally serum albumin), and lysozyme, proteolytic (trypsin-like), and myeloperoxidase activities were detected in lavage fluids obtained from animals instilled with silica. These factors (except for lysozyme activity) were elevated above control levels from 3 to 7 days postinstillation and declined to near control levels by Day 14. The rate of DNA, collagen, and noncollagen protein synthesis was significantly elevated in lung tissue minces from silica-treated rats 3 and 7 days after instillation. Elevated levels of total protein, and lung collagen in particular, were observed 9 weeks after insult. Lavage fluid from silica-instilled rats stimulates DNA synthesis in cultures of proliferating and quiescent rat lung fibroblasts. Lavage fluid from silica-instilled rats also stimulates lung fibroblasts to increase collagen and noncollagen protein synthesis.