Objectives: Angiotensin II (Ang II) induces fibroblast proliferation and collagen synthesis in the myocardium, but its precise mechanisms of action in human hearts are still unknown. Therefore, we investigated whether Ang II directly affects the collagen mRNA content in the human myocardium and in isolated human cardiac fibroblasts or whether the growth factors TGFβ-1 and osteopontin are involved in this process. Methods and results I: In a first set of experiments, the direct effect of Ang II on collagen I, TGFβ-1 and osteopontin mRNA content in fresh samples of human atrial myocardium was determined by the use of a short stimulation period. After 4 h, Ang II-stimulated atrial samples gave a significantly higher expression of both TGFβ-1 (183±21% of control, p<0.05) and osteopontin mRNA (275±58%, p<0.02) than the controls. In contrast, the expression of collagen I mRNA was unchanged (95±8%). Stimulation with TGFβ-1 led to an increase in collagen I and III mRNA (127±10%, p<0.05; 140±15%, p<0.02). Methods and results II: In a second protocol, to assess the effects of longer stimulation periods, we determined the effects of Ang II and its potential mediator TGFβ-1 on collagen I, III and fibronectin mRNA expression and on proliferation of cultured human cardiac fibroblasts. Ang II caused a dose-dependent stimulation of proliferation but did not affect collagen I, III or fibronectin mRNA content after 24 h. In contrast, TGFβ-1 stimulation significantly increased collagen I and III mRNA expression (124±5% and 128±5%, p<0.002). Conclusions: In the human heart, Ang II does not directly increase collagen or fibronectin mRNA, but it does increase TGFβ-1 and osteopontin mRNA expression. Since TGFβ-1 induces collagen I and III mRNA in atrial samples and in isolated cardiac fibroblasts, it may represent a necessary mediator of the Ang II effects in the human heart.