We investigated whether acteoside can inhibit mesangial matrix expansion or mesangial cell proliferation in mesangioproliferative anti-Thy 1 nephritis. Untreated control rats were compared to the rats treated with acteoside either during early (day 1 to 8) or late (day 4 to 12) period after the induction of anti-Thy 1 nephritis. The result showed that acteoside and prednisolone treatments (in either early or late period) significantly reduced proteinuria, mesangial matrix expansion (the index of matrix expansion) and mesangial proliferation as determined by the number of proliferating nuclear cell antigen (PCNA)-positive cells. Acteoside also reduced glomerular macrophage infiltration and ICAM-1 expression in glomeruli of anti-Thy 1 nephritic rats. Furthermore, acteoside treatment markedly increased the activities of matrix metaloproteinases (MMP) in glomeruli. These results suggest that acteoside can inhibit mesangial cell proliferation and extracellular matrix overproduction by either inhibiting ICAM-1 expression or increasing activities of MMP.