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Total saponins of Panax notoginseng (PNS) have been shown to ameliorate renal interstitial fibrosis. Ginsenoside Rg1, a panaxatriol saponin, is one of the major active molecules from PNS. The present study was undertaken to investigate the effect of ginsenoside Rg1 on renal fibrosis in rats with unilateral ureteral obstruction (UUO). The rats were randomly divided into 3 groups: sham-operation (n=15), UUO (n=15) and UUO with ginsenoside Rg1 treatment (n=15, 50 mg per kg body weight, intraperito- neally (i.p.) injected). The rats were sacrificed on Days 7 and 14 after the surgery. Histological examination demonstrated that ginsenoside Rg1 significantly inhibited interstitial fibrosis including tubular injury as well as collagen deposition. α-smooth muscle actin (α-SMA) and E-cadherin are two markers of tubular epithelial-myofibroblast transition (TEMT). Interestingly, ginsenoside Rg1 notably decreased α-SMA expression and simultaneously enhanced E-cadherin expression. The messenger RNA (mRNA) of transforming growth factor-β1 (TGF-β1), a key mediator to regulate TEMT, in the obstructed kidney increased dra- matically, but was found to decrease significantly after administration of ginsenoside Rg1. Further study showed that ginsenoside Rg1 considerably decreased the levels of both active TGF-β1 and phosphorylated Smad2 (pSmad2). Moreover, ginsenoside Rg1 substantially suppressed the expression of thrombospondin-1 (TSP-1), a cytokine which can promote the transcription of TGF-β1 mRNA and the activation of latent TGF-β1. These results suggest that ginsenoside Rg1 inhibits renal interstitial fibrosis in rats with UUO. The mechanism might be partly related to the blocking of TEMT via suppressing the expression of TSP-1.