Abstract The metabolic phases of increased fibrogenesis were studied by determining the activity of lactate dehydrogenase (LDH) and its isoenzyme pattern in the lung tissue of silicotic rats. Silicosis, induced by intratracheal injection of silica particles, caused increased collagen formation, measured as the hydroxyproline content of the lung or evaluated histochemically. Total LDH activity was found to be increased after 3 weeks and had doubled at 5 weeks after the onset of silicosis. This increase was found to be due mainly to LDH-5. These results suggest that the early events, preceding collagen accumulation in the lung tissue, consist in metabolic adaptation to anaerobic conditions.