Abnormal stiffness and altered cardiac function arising from abnormal collagen deposition occur in hypertrophy and heart failure. Ang II has been shown to play a role in this process. To evaluate the mechanism, we developed an in vitro model by subjecting fibroblasts to angiotensin II (Ang II) treatment in the presence or absence of myocytes in coculture (25). Employing this model, we demonstrated that Ang II-induced collagen gene transcription in cardiac fibroblasts was potentiated by myocyte-derived factors. In attempting to identify mechanisms of collagen upregulation and to define the role of myocytes, we found that interleukin 6 (IL-6), tumor necrosis factor alpha (TNF-), and the transforming growth factor beta (TGF-) superfamily were also involved in collagen upregulation. Collagen transcripts were increased after fibroblasts were treated with IL-6 (20 to 50 ng/ml) and TNF- (0.1 to 0.5 ng/ml). In this study we show that cardiomyocytes induce secretion of active TGF- in the presence of Ang II and that a paracrine action of TGF- subsequently induces different cytokines (IL-6) in fibroblasts, thereby promoting collagen synthesis. The crosstalk between myocytes and fibroblasts and involvement of these cytokines in upregulation of collagen transcript levels are novel findings that may explain their possible roles in the upregulation of collagen.