We have studied the inhibitory effects of Desmodium styracifolium-triterpenoid (Ds-t) (extracted from Desmodium styracifolium (Osbeck) Merr, a herbal medicine) on the formation of calcium oxalate renal stones induced experimentally by ethylene glycol (EG) and 1 alpha(OH)D3 (1 alpha D3) in rats. The incidence of urinary stone formation was 81% in the control group, which received EG and 1 alpha D3, and 29% in the Ds-t group, which received EG and 1 alpha D3 supplemented by Ds-t. The serum calcium (Ca) concentration in the Ds-t group was significantly elevated and urinary Ca excretion was markedly reduced. Urinary excretion of citrate (Cit), a factor that prevents stone formation, was significantly increased in the Ds-t group. Excretion of urinary phosphorus (P), which was elevated to a significantly greater extent in the controls than in the Ds-t group, was increased in both groups. The increase in urine volume in the Ds-t group was significantly greater than in the control group. The 24-h creatinine clearance rate (Ccr) was significantly lower in the controls. These findings suggest that Ds-t inhibits the formation of Ca oxalate stones in rat kidneys by increasing the output of urine, decreasing the excretion of calcium and increasing the urinary excretion of citrate. Ds-t may be useful in preventing the recurrence of urinary Ca oxalate stones in the clinical setting.