Leaves of Perilla frutescens var. crispa DECNE. (perilla, Labiatae) are used as a garnishing vegetable in East Asian countries as well as an herbal medicine prescribed in Kampo medicines such as Saiboku-to. A previous in vitro study revealed that a decoction of perilla leaves inhibits the proliferation of murine-cultured mesangial cells. In the present study, we evaluated the in vivo anti-proliferative effects of a perilla decoction using rat mesangio-proliferative glomerulonephritis induced by an intravenous injection of rabbit anti-rat thymocyte serum (ATS). Leaves of perilla were boiled, and the decoction was orally administered to the rats as drinking water at doses of 100 and 500 mg/kg/d from the day of ATS-injection (day 0) to day 8, when rats were sacrificed. In the histological evaluation, the total number of glomerular cells, proliferating cell nuclear antigen (PCNA) positive cells, and macrophage/monocyte antigen-positive cells in the glomerulus, was significantly decreased in perilla-treated rats. A significantly lower level of proliferation was induced by the serum of the perilla-treated rats than by that of the controls. These results suggest that the perilla decoction suppresses the proliferation of mesangial cells in vivo by an inhibition of the glomerular infiltration of macrophage/monocytes and of the production of circulating growth factors.