Cimadronate (YM175) is a novel bisphosphonate with potent inhibitory activity on bone resorption under development for the treatment of tumor-induced hypercalcemia, metastatic bone disease and osteoporosis. We conducted toxicity studies of cimadronate intravenously, single dose, 30-day repeated dose and 26-week weekly dose in F344 rats. In the single dose study, cimadronate was administered to rats and the animals were observed for 14 days. Major toxic symptoms were decreased motility and piloerection and LD50 values were 23 mg/kg for males and 21 mg/kg for females. In the 30-day study, the animals received cimadronate at doses of 0, 0.16, 0.31, 0.62 or 1.25 mg/kg/day. At 0.16 mg/kg/day or more, an increased amount of primary spongiosa was observed in the femur. At 0.62 mg/kg/day or more, renal and testicular/epididymal toxicity were observed. After a 30-day recovery period, the finding in the kidney disappeared, but the findings remained in the bone, testis and epididymis. In the 26-week weekly dose study, animals received cimadronate at doses of 0, 0.31, 0.62 and 1.25 mg/kg/week. At 0.31 mg/kg/week or more, an increased amount of primary spongiosa was seen in the femur. Renal and testicular/epididymal toxicity, however, were not observed.