Matrix metalloproteinases (MMPs) are the principle enzymes that initiate degradation of collagen. We examined the role of MMPs during alveolar wall fibrosis and fibrotic nodule formation due to silica exposure. Rats were exposed to filtered air or 15 mg/m³ silica by inhalation for 5 days/week, 6 hours/day. Lungs were preserved by intra-tracheal instillation (IT) of fixative at 20, 40, 60, 79 and 116 days of exposure. Additional groups were fixed after 20, 40 and 60 days of exposure followed by 36 days of recovery. The number of nodules, defined by a collagenous core and a bounding cell layer detached from the alveolar wall, was determined by morphometry. Lungs showed an increase in alveolar wall collagen and fibrotic nodules at 79 and 116 days of exposure with increased collagenase and gelatinase activity. The number of nodules per lung in exposed groups increased from 619±447 at 40 days to 13221±1096 at 116 days (mean ±SE,n=5). No nodules were seen in control lungs. Silica-exposed rats with a 36-day recovery in filteredair showed an enhanced MMP activity over exposure to silica for the same duration with no recovery. MMP-2 and MMP-9 were significantly elevated in alveolar macrophages after 40 days of exposure. Stromelysin expression was demonstrated in alveolar macrophages and cells within fibrotic nodules. TIMP-1 expression was not significantly altered. In summary, MMP activity was upregulated at 40 days of silica exposure and progressively increased during the ensuing fibrotic responses. Early expression of stromelysin was found in fibrosing alveolar walls and fibrotic nodules.