Cardiac fibroblasts appear to be important in producing and maintaining the extracellular matrix (ECM) of the heart. The abnormal proliferation of cardiac fibroblasts and deposition of the ECM protein, collagen, associated with hypertension and myocardial infarction, may adversely affect the performance of the heart. Several groups of factors affect collagen gene expression and/or growth of cardiac fibroblasts. Angiotensin II, aldosterone and endothelins play a central role in the remodeling of the ECM in hypertension, and decrease collagenase activity and/or increase collagen synthesis in cultured cells. Regulatory peptides that are generally elevated at sites of injury, such as TGF-β₁ and PDGF, increase collagen synthesis and/or stimulate mitogenesis. Mechanical stretch enhances collagen expression and cell proliferation, responses which could in part be due to integrin activation. Cytokines may stimulate or inhibit cell growth, the latter through prostaglandin formation. Angiotensin II is a principal determinant in vivo of cardiac fibroplasia and synthesis of the ECM proteins, collagen and fibronectin. Cardiac fibroblasts possess G-protein-coupled AT₁ receptors for angiotensin II that couple to activation of multiple signalling pathways, including: phospholipase C-β, with the subsequent release of Ca²⁺ from intracellular stores and activation of protein kinase C, mitogen-activated protein kinases, tyrosine kinases, phospholipase D, phosphatidic acid formation, and the STAT family of transcription factors. Cardiac fibroblasts respond to angiotensin II with hyperplastic/hypertrophic growth, and increased expression of collagen, fibronectin, and integrins. The mechanisms by which the AT₁ receptor activates multiple signalling pathways are not known, although the receptor might interact at some level with both integrins and cytokine receptors. Different signalling pathways of the AT₁ receptor may subserve different cellular responses, such as mitogenesis, ECM synthesis, or an inflammatory/stress response. Crosstalk among the signalling pathways of the AT₁ receptor, and those of G-protein, cytokine, and growth-factor receptors, may determine the ultimate response of the cell.