Impaired autoregulation in chronic kidney disease can result in elevation of glomerular capillary pressure and progressive glomerular damage; however, the factors linking chronic glomerular disorders to impaired autoregulation have not been identified. We tested the hypothesis that the cytokine most closely associated with progressive glomerular disease, transforming growth factor (TGF)-β, may also attenuate autoregulation. Kidneys from normal rats were prepared for videomicroscopy, using the blood-perfused juxtamedullary nephron technique. Autoregulatory responses were measured under control conditions and during superfusion with TGF-β1 (10 ng/ml). Control afferent arteriolar diameter averaged 18.4 ± 1 μm and significantly decreased to 16.3 ± 0.9 and 13.2 ± 0.8 μm at perfusion pressures of 130 and 160 mmHg, respectively. In the presence of TGF-β1, autoregulatory responses were completely blocked. In similar experiments performed using PDGF-BB (10 ng/ml) and HGF (25 ng/ml), the normal autoregulatory response was not affected. In vitro studies, using isolated preglomerular vascular smooth muscle cells, revealed that exposure to TGF-β1 stimulated a rapid increase in reactive oxygen species (ROS) that was inhibited by NADPH oxidase inhibitors. In situ studies, with dihydroethidium staining, revealed a marked increase in renal vessel ROS production on exposure to TGF-β1. Pretreatment of the juxtamedullary afferent arterioles with tempol, a ROS scavenger, or with apocynin, a NADPH oxidase inhibitor, prevented the impaired autoregulation induced by TGF-β1. These data reveal a novel hemodynamic pathway by which TGF-β could lead to progressive glomerular injury by impairing normal renal microvascular function. Copyright © 2005 the American Physiological Society.