Intermittent treatment with human parathyroid hormone (1-34) ] stimulates bone formation and increases cancellous bone mass in ovariectomized (OW) rats. But PTH-induced cancellous bone rapidly disappears upon cessation of treatment. The fate of cortical bone treated by PTH has not been well characterized. Incadronate disodium (disodium cycloheptylaminomethylenedisphosphonate monohydrate, YM 175) was expected to be antiresorptive without inhibiting bone formation. The purposes of this study were to determine (1) whether PTH treatment increases new cancellous and cortical bone mass and bone formation, (2) whether the new bone could be maintained by YM175 sequential treatment, and (3) whether the maintenance effect is persistent after YM 175 withdrawal. Eighty-eight 1 1-week-old Sprague-Dawley rats were divided into sham operation and O W groups. The OVX rats were treated for 8 weeks with the subcutaneous intermittent injection of 30 &kg of hPTH( 1-34) three times a week beginning 4 weeks after surgery, then PTH treatment was withdrawn and YM175 (10 &kg) was injected subcutaneously three times a week for 4 weeks. YM175 treatment was withdrawn for the last 8 weeks of the protocol. The results of microstructural assessment in proximal tibial metaphysis and bone mineral density in distal and proximal femur demonstrated that PTH treatment for 8 weeks restored bone mass to the sham control level. However, after cessation of PTH treatment, the PTH-induced tibial cancellous bone mass showed a decrease at 4 weeks and almost totally disappeared after 12 weeks. Conversely, YM175 treatment maintained the PTH-induced tibial cancellous bone mass, and the bone continued to be maintained after 8 weeks of withdrawal of the YM175. Cortical bone was not lost during PTH treatment. YM175 maintained the PTHinduced new tibial cancellous bone in OVX rats by suppressing remodeling.