INTRODUCTION: Thy1.1 glomerulonephritis (Thy1.1 GN) in rats is widely used as an experimental model of mesangial proliferative glomerulonephritis (GN). We previously reported that T-helper (Th) cells were accumulated in glomeruli from the early phase of this model and that not Th2 cells but Th1 cells play an important role in the development of glomerular alterations. Although Th17 is reported to be involved in the pathogenesis of several autoimmune diseases, the role of Th17 cells in the pathogenesis of mesangial alterations in Thy1.1 GN remains unclear. METHODS: The kinetics of the infiltration of subsets of Th cells and the expression of IL-17 in Thy1.1 GN were analyzed. Next, the localization and the cell types of IL-17 receptor (IL-17R)-positive cells and IL-6-positive cells were analyzed. Then, the effect of tacrolimus on the expressions of Th17-related cytokines in Thy1.1 GN was analyzed. RESULTS: Not only Th1 cells but also Th17 cells were recruited into glomeruli from the early phase of the disease. mRNA expression of IL-17 in glomeruli was elevated. The increased positive expression of IL-17R was detected in the mesangial area, and some of IL-17R-positive cells were co-stained with IL-6. Tacrolimus treatment ameliorated mesangial alterations by suppressing the expressions of Th17-related cytokines such as IL-17 and IL-6. CONCLUSION: Th17 cells participate in the development of Thy1.1 GN, a mimic of mesangial proliferative GN, and Th17 cells and their related cytokines are pertinent therapeutic targets.