The effects of saikosaponin-d extracted from the roots of Bupleurum falcatum L. on aminonucleoside nephrosis were studied in rats. Urine protein excretion in rats receiving aminonucleoside alone was significantly elevated on the 2nd day after the last injection of aminonucleoside and reached a peak on the 11th day. Urinary protein on the 11th day was reduced by 48 and 46%, respectively in animals treated with saikosaponin-d from the 2nd and 8th day after the last injection of aminonucleoside. Electron microscopically, the degree of abnormality, e.g. loss or fusion of foot processes, in the glomerular epitherial cells was significantly lower in the rats treated with saikosaponin-d after aminonucleoside injection than in the rats treated with aminonucleoside alone. It is concluded that saikosaponin-d prevents the development of proteinuria induced by aminonucleoside in the rat.