Pulmonary basement membranes are believed to play important roles in lung morphogenesis, in maintenance of lung alveolar architecture, and in repair after pulmonary injury. However, very little is known about the synthesis and matrix deposition of lung basement membrane macromolecules. Accordingly, we have investigated the synthesis of type IV procollagen, the major collagenous component of basement membranes, in slices of adult rat lung. After a 4-h labeling with [3H]proline, type IV procollagen chains were extracted from lung homogenates with 2 M guanidine-HCl and purified by salt fractionation and ion exchange chromatography. The chains comigrated with authentic type IV chains by SDS-PAGE and were selectively coprecipitated with antibodies to type IV collagen. Peptide mapping confirmed their identity as pro alpha 1(IV) and pro alpha 2(IV), and suggested that the chains are predominantly assembled as heteropolymers. There was no evidence for proteolytic processing of the newly synthesized type IV procollagen even though type I and III procollagens were rapidly processed to lower molecular weight intermediates and collagen. Type IV procollagen accounted for approximately 40% of the extracted radiolabeled collagen, suggesting that there may be a relatively high turnover of lung basement membrane collagen in vivo.