—We tested the hypothesis that the inflammatory cytokines can regulate fibroblast extracellular matrix metabolism. Neonatal and adult rat cardiac fibroblasts cultures in vitro were exposed to interleukin (IL)–1β (4 ng/mL), tumor necrosis factor-α (TNF-α; 100 ng/mL), IL-6 (10 ng/mL), or interferon-γ (IFN-γ; 500 U/mL) for 24 hours. IL-1β, and to a lesser extent TNF-α, decreased collagen synthesis, which was measured as collagenase-sensitive [³H]proline incorporation, but had no effect on cell number or total protein synthesis. IL-1β decreased the expression of procollagen α₁(I), α₂(I), and α1(III) mRNA, but increased the expression of procollagen α₁(IV), α₂(IV), and fibronectin mRNA, indicating a selective transcriptional downregulation of fibrillar collagen synthesis. IL-1β and TNF-α each increased total matrix metalloproteinase (MMP) activity as measured by in-gel zymography, causing specific increases in the bands corresponding to MMP-13, MMP-2, and MMP-9. IL-1β increased the expression of proMMP-2 and proMMP-3 mRNA, suggesting that increased metalloproteinase activity is due, at least in part, to increased transcription. The effects of IL-1β were not dependent on NO production. Thus, IL-1β and TNF-α decrease collagen synthesis and activate MMPs that degrade collagen. These observations suggest that IL-1β and TNF-α may contribute to ventricular dilation and myocardial failure by promoting the remodeling of interstitial collagen.