We examined the bone turnover and bone mass in adjuvant-induced arthritis in rats and assessed the effects of indomethacin in this model. One hundred ten SD rats, 6 weeks of age, were assigned to 11 groups and injected with adjuvant or solvent in the right foot. Adjuvant-injected rats were orally administered indomethacin at doses of 0 (vehicle), 0.1 (low), 0.5 (medium), and 1.5 (high) mg/kg body weight from the start (day 0). Animals were sacrificed on days 0, 14 (acute phase), and 28 (chronic phase). In the arthritic-control group, serum osteocalcin level and bone mineral content of the fourth lumbar body (L4) and the femur were significantly reduced on day 14. Serum alkaline-phosphatase was increased on day 28. Trabecular bone volume of L4 was decreased on day 14, and the value was further decreased on day 28. Bone formation rate (BFR/BS) was significantly reduced on day 14, and then osteoclast number (Oc.N/BS) increased on day 28. Indomethacin treatment dose-dependently prevented increases in paw volume and osteoclast number. In the high dose group, these indices were maintained at the same level with those in the normal group. However, indomethacin treatments were not able to maintain the parameters of bone formation such as serum osteocalcin and BFR/BS values, and the trabecular bone mass decrease was only partially prevented. These data clearly indicated both reduced bone formation and increased bone resorption as the causes of bone loss in adjuvant-induced arthritis in rats. Increased bone resorption seemed to be due to the increased activity of prostaglandins, but bone formation defect would be related to other factors in this animal model.