Previous studies have demonstrated that angiotensin II (ANG II) mediates cell cycle arrest of cultured renal tubular cells by induction of p27(Kip1), an inhibitor of cyclin-dependent kinases. However, it is not known whether ANG II exerts similar effects in vivo. Infusion of ANG II into naive rats for 7 days increased formation of reactive oxygen species in tubular cells of the kidney. Furthermore, ANG II infusion stimulated protein expression of p27(Kip1) as detected by western blotting of tubular lysates and immunohistochemistry. Infusion of ANG II reduced tubular proliferation as detected by proliferating-cell nuclear antigen (PCNA) immunohistochemistry. The increase in p27(Kip1) expression was not due to an increase in mRNA. Immunoprecipitation experiments revealed that the increased p27(Kip1) protein associates with cyclin-dependent kinase 2. Coadministration of the radical scavenger dimethylthiourea abolished this ANG II mediated p27(Kip1) expression without reducing systemic blood pressure. Furthermore, dimethylthiourea infusion attenuates the ANG II mediated G(1)-phase arrest of tubular cells. However, infusion of norepinephrine did not induce reactive oxygen species or p27(Kip1) expression, despite a significant increase in blood pressure. Thus ANG II induces p27(Kip1) expression in renal tubular cells in vivo. This effect is mediated by reactive oxygen species. Since tubular hypertrophy depends on G(1)-phase arrest and may promote subsequent development of interstitial fibrosis, administering oxygen radical scavenger may be a therapeutic tool to counteract ANG II dependent remodeling of renal tubular cells.