This study was performed to examine the effects of the antifibrotic agents TJN-331 and tranilast on mesangial expansion in a rat model of anti-Thy1 nephritis. We first investigated the effects of TJN-331 and tranilast on mesangial expansion induced by anti-Thy1 serum in rats, and determined the counts of glomerular cells and proliferative cell nuclear antigen (PCNA)-positive cells. The effects of TJN-331 and tranilast on production of transforming growth factor-β1 (TGF-β1) by isolated glomeruli incubated for 48 h were then examined. The TGF-β1 staining score, the number of TGF-β1-positive cells and the TGF-β1 receptor-positive area in the anti-Thy1 nephritis model were also measured using immunohistochemistry. TJN-331 administered from day 1 (the day after anti-Thy1 serum injection) blocked an increase in mesangial matrix accumulation on days 4 and 8, compared to untreated anti-Thy1 nephritic rats. TJN-331 also inhibited both the increase in the number of glomerular cells on day 8 and the decrease in this cell count on day 2 observed in untreated nephritic rats, and TJN-331 and tranilast inhibited an increase in PCNA-positive cells in the glomerular cross section on days 4 and 8. Both TJN-331 and tranilast inhibited increases in the TGF-β1 protein content from nephritic glomeruli, the TGF-β1-positive area, and the number of TGF-β1-positive cells/cross section in anti-Thy1 nephritic glomeruli. These results suggest that TJN-331 and tranilast prevent expansion of the mesangial area by suppression of TGF-β1 secretion from inflamed glomeruli.