Background Urotensin Ⅱ (UⅡ) is a new vasoconstrictive peptide that may activate the adventitial fibroblasts.Transforming growth factor-β1 (TGF-β1) is an important factor that could induce the phenotypical transdifferentiation of adventitial fibroblasts. This study aimed to explore whether TGF-β1 is involved in UⅡ-induced phenotypic differentiation of adventitial fibroblasts from rat aorta.Methods Adventitial fibroblasts were prepared by the explant culture method. TGF-β1 protein secretion from the cells was determined by enzyme-linked immunosorbent assay (ELISA). The mRNA and protein expression of α-smooth nuscle actin (α-SM-actin), the marker of phenotypic differentiation from fibroblasts to myofibroblasts, were determined using real-time quantitative RT-PCR (real-time RT-PCR) and West blotting, respectively.Results UⅡ stimulated the secretion of TGF-β1 in cultured adventitial fibroblasts in a time-dependent manner. The secretion reached a peak at 24 hours, was higher by 69.8% (P ＜0.01), than the control group. This effect was also concentration dependent. Maximal stimulation was reached at 10-8 mol/L of UⅡ (P ＜0.01), which was increased by 59.9%,compared with in the control group (P ＜0.01). The secretion of TGF-β1 induced by UⅡ was significantly blocked by SB-710411 (10-7 mol/L), a specific antagonist of UⅡ receptor. In addition, both UⅡ (10-8 mol/L) and TGF-β1 significantly stimulated α-SM-actin mRNA and protein expression. Moreover, the α-SM-actin induced by UⅡ was inhibited by the specific neutralizing antibody (20 μg/ml) of TGF-β1, while the α-SM-actin expression stimulated by TGF-β1 (20 ng/ml)was inhibited by SB-710411 (10-7 mol/L), the UⅡ receptor antagonist.Conclusion This study suggests that UⅡ could induce TGF-β1 secretion in adventitial fibroblasts via UT activation, and TGF-β1 might be involved in phenotypic differentiation from adventitial fibroblasts into myofibroblasts induced by UⅡ, and TGF-β1 signaling might be one of the important pathways by which UⅡ is involved in vascular fibrosis.