Aims: The NADPH oxidases constitute a major source of superoxide anion (·O2¿) in hypertension. Several studies suggest an important role of NADPH oxidases in different effects mediated by transforming growth factor-ß1 (TGF-ß1). We investigated whether a chronic treatment with P144, a peptide synthesized from type III TGF-ß1 receptor, inhibited NADPH oxidases in the renal cortex of spontaneously hypertensive rats (SHR). Results: Here, we show that chronic administration of P144 significantly reduced the NADPH oxidase expression and activity as well as the oxidative stress observed in control vehicle-treated SHR (V-SHR). In addition, P144 was also able to reduce the significant increase in the renal fibrosis and in mRNA expression of different components of collagen metabolism, as well as in the levels of connective tissue growth factor observed in V-SHR. Finally, TGF-ß1-stimulated NRK52E exhibited a significant increase in NADPH oxidase expression and activity as well as a TGF-ß1-dependent intracellular pathway that were inhibited in the presence of P144. Innovation: Our experimental evidence suggests that reversing oxidative stress may be therapeutically useful in preventing fibrosis-associated renal damage. We show here that (i) the TGF-ß1-NADPH oxidases axis is crucial in the development of fibrosis in an experimental hypertensive renal disease animal model, and (ii) the use of P144 reverses TGF-ß1-dependent NADPH oxidase activity; thus, P144 may be considered a novel therapeutic tool in kidney disease associated with hypertension. Conclusion: We demonstrate that P144 inhibits NADPH oxidases and prevents oxidative stress in kidneys from hypertensive rats. Our data also suggest that these effects are associated with the renal antifibrotic effect of P144.