Overexpression of transforming growth factor beta (TGF-beta) has been shown to play pathogenic roles in progression of renal fibrosis, and the severity of tubulointerstitial fibrosis correlates better with renal function than the severity of glomerulosclerosis. Smad proteins are signaling transducers downstream from TGF-beta receptors. Three families of Smad proteins have been identified: receptor-regulated Smad2 and Smad3, common partner Smad4, and inhibitory Smad7 (part of a negative-feedback loop). We investigated Smad-mediated TGF-beta signaling pathway and regulatory mechanisms of inhibitory Smad7 in unilateral ureteral obstruction (UUO) kidneys in mice, a model of progressive tubulointerstitial fibrosis. Compared with sham-operated kidneys, the level of Smad7 protein, but not mRNA, decreased progressively in UUO kidneys, whereas immunoreactivity for nuclear phosphorylated Smad2 and Smad3 and renal fibrosis were inversely increased. Furthermore, we demonstrated that both the degradation and ubiquitination activity of Smad7 protein were increased markedly in UUO kidneys compared with sham-operated ones. We also found that both Smurf1 and Smurf2 (Smad ubiquitination regulatory factors), which are E3 ubiquitin ligases for Smad7, were increased and that they interacted with Smad7 in UUO kidneys. Our results suggest that the reduction of Smad7 protein resulting from enhanced ubiquitin-dependent degradation plays a pathogenic role in progression of tubulointerstitial fibrosis.