Aging lung is characterized by morpho-structural modifications, including progressive fibrosis, that lead to an altered function. Here we provide a comprehensive description of lung collagen expression and metabolism during natural aging of rats. Peribronchial collagen increased significantly in the oldest animals (p=0.05 2- vs. 6- and 19-month-old rats), as a consequence of Collagen-I and Collagen-III (COL-I, COL-III) protein accumulation (p<0.05 in 6-, 12- and 19-month-old rats versus the youngest). No changes in fibronectin (FN) protein expression and in COL-III and transforming grow factor beta-1 (TGFbeta-1) mRNA expression were observed. Conversely the transcription activity of the COL-I gene was overexpressed in the oldest animals (p<0.05). In the aged rats, the activity of lung matrix metalloproteinases (MMP), MMP-1 and MMP-2, dropped significantly (p<0.05), whilst MMP-9 levels were slightly decreased. These changes were associated with a concomitant increase of tissue inhibitors of MMP (TIMP-1 and TIMP-2). All together, these results suggest that, during natural aging, collagen accumulation in the lung and its progressive fibrosis are mainly due to a reduced proteolytic activity of MMP, in which TIMP-1 and -2 seem to be the major regulating factors.