Angiotensin (Ang) II is considered the effector peptide of the renin-angiotensin system (RAS) that acts as a renal growth factor. Some studies have shown that the angiotensin degradation product Ang III presents some biological activities, though its role in renal pathology has not been explored. We have observed that in renal interstitial fibroblasts Ang III induces c-fos gene expression, suggesting a potential role of Ang III in the control of cell proliferation. To study the involvement of Ang III in matrix regulation, we determined whether Ang III increased TGF-beta gene expression and fibronectin production in cultured rat mesangial cells and renal interstitial fibroblasts, the main effector cells in glomerular and interstitial fibrosis, respectively. In both cell types, treatment with Ang III (10(-7) M) for six hours up-regulated gene expression of transforming growth factor-beta 1 (TGF-beta 1; 2.3- and 2.2-fold, respectively). This peptide also increased fibronectin production in renal interstitial fibroblasts. All these data suggest that Ang III could contribute to matrix accumulation. Activation of local RAS has been described during renal damage. Renal cells express angiotensinogen mRNA that was up-regulated in response to Ang II and Ang III stimulation, and therefore both peptides may participate in the generation of angiotensin peptides in the kidney. In conclusion, our results suggest that the angiotensin degradation product Ang III could participate in the pathogenesis of key events of renal diseases, supporting the hypothesis that other peptides of the RAS besides Ang II may be involved in renal injury.