Elastin, an important structural protein of the extracellular matrix, confers elastic properties on the pulmonary alveolar interstitium. In the alveolar wall, elastin is primarily produced postnatally by fibroblasts. The mechanisms that regulate lung fibroblast (LF) elastin gene expression have not been completely defined, although both transcriptional and posttranscriptional mechanisms appear to be involved. Transforming growth factors-beta (TGF-beta s) have been shown to increase elastin production by cultured neonatal rat LF. Analyses of elastin gene transcription and mRNA stability indicate that exogenous TGF-beta 1 increases the half-life of tropoelastin mRNA by 1.5-fold and does not alter elastin gene transcription. Interference with the functions of endogenous TGF-beta 1 in cultured LF, through the addition of neutralizing antibodies or antisense oligodeoxynucleotides, decreases tropoelastin and tropoelastin mRNA production by these cells. The content of total (latent plus active) TGF-beta s was approximately 4.5-fold greater in lungs obtained from rats on postnatal day 8 than in lungs obtained from adults. These findings indicate that endogenous TGF-beta s, in cultured LF, regulate elastin gene expression, most likely by a posttranscriptional mechanism. Since others have shown that elastin mRNA appears to have a longer half-life in neonatal than in adult rat lungs, we hypothesize that the higher content of TGF-beta s could contribute to the greater elastin mRNA stability in neonatal lungs.