Previous reports have shown that bone mass and architecture will partially recover by remobilization (RM) in immobilization (IM)-induced osteopenia. The aim of this study was to test whether PTH can accelerate the recovery during RM from the IM-induced osteopenia. Six-month-old Sprague-Dawley female rats were divided into aging and IM groups. The right hindlimb of rats was immobilized against the abdomen by elastic bandages for 18 weeks, then groups of rats were further IM or RM for 2, 10 and 20 weeks and given 30 or 80 micrograms hPTH (1-38)/kg/d s.c. Secondary spongiosa of proximal tibial metaphyses (PTM) were studied. Immobilization reduced the trabecular area, number and thickness at the first 18 weeks post IM, then plateaued. Ten weeks of RM restored 40% trabecular bone from IM controls due to thickening of the trabeculae. However, the trabecular area was still -14% and -13% lower than that of aging controls at 10 and 20 weeks. Two weeks of 30 micrograms PTH/kg/d in IM rats restored the bone to aging control levels, two weeks of 30 micrograms PTH/kg/d in RM rats and of 80 micrograms PTH/kg/d in both RM and IM rats added extra bone. Extending the treatment to 10 and 20 weeks, the same amount of total bone was added to both IM and RM rats. These findings indicate PTH is a powerful agent that can accelerate the recovery and add extra bone to osteopenic cancellous bone in both IM and RM rats.