Rat lung collagen was labelled in vivo by a single intraperitoneal injection of [3H]lysine at several key timepoints in lung development: days 11 (alveolar proliferation), 26 (start of equilibrated growth), 42 (end of equilibrated growth), and 100 (adult lung structure present). The rates of deposition of labelled hydroxylysine and the difunctional, Schiff base-derived crosslinks hydroxylysinonorleucine (HLNL) and dihydroxylysinonorleucine (DHLNL) were quantified. We also measured total lung content of the trifunctional, mature crosslink hydroxypyridinium (OHP) in these same animals. While the relative rates of accumulation of labelled collagen [3H]hydroxylysine differed by a factor of about 6 at the different times of injection of labelled precursor, quantitative and qualitative patterns of collagen crosslinking were very similar at all of the lung developmental stages studied. Furthermore, there was little or no breakdown of the lung collagen pool as defined by the presence of labelled crosslinks; changes in lung DHLNL content could be completely accounted for by its maturation to OHP, regardless of the age of the rats when injected with the radioactive precursor. We conclude that mature, crosslinked collagen in the lungs of rats, which is obligatorily an extracellular pool, is not being degraded at a measurable rate. Therefore, studies of others that have shown apparent high rates of breakdown of newly synthesized collagen in lungs of whole animals using different methods are probably not reflective of the metabolic fate of total lung collagen, and may indicate that degradation of normal lung collagen occurs predominantly or exclusively intracellularly.