The effects of boron alone or in combination with estrogen or parathyroid hormone (PTH) on bone quality and Ca, P, and Mg metabolism were investigated in 12- week-old rats. Treatment began on day 43 after ovariectomy and continued for 5 weeks. Treatments included 5 ppm (5 mg per kg diet) boron as boric acid, 17β estradiol (30 μg/kg/d, s.c.), PTH (rat PTH 1-34 fragment, 60 μg/kg/d, s.c.), boron+17β estradiol, and boron+PTH at the same dose. All rats had free access to food (AIN 76) and deionized water. Ovariectomy caused decreases in the ratios of dried bone, bone ash, bone Ca, P, and Mg to body or dried bone weight, wall thickness of the femur, urinary Ca/creatinine excretion, trabecular bone volume, bone surface density, and trabecular plate density, and increases in serum osteocalcin, and trabecular plate separation. However, intermittent PTH injection increased the dried bone weight to body weight ratio, wall thickness of the femur, total bone ash, bone ash to dried bone or body weight ratio, bone Ca, P, and Mg, bone Ca, Mg, and P to dried bone weight or body weight ratios, trabecular bone volume, bone surface density, trabecular plate density, and thickness, serum osteocalcin concentration, and decreased trabecular plate separation, but had no effects on Ca, P, and Mg metabolism. Treatment with 17β estradiol increased the dried bone weight, bone ash, and bone Ca, P, and Mg to body weight ratios, and decreased serum osteocalcin, and trabecular bone separation, but had no effects on Ca, P, and Mg metabolism. However, when combined with boron, 17β estradiol not only remained the positive effects as observed in Ovx rats treated with 17β estradiol alone, but also improved apparent Ca, Mg, and P absorption, and Ca and Mg retention, trabecular bone volume, bone surface density and trabecular plate density which were not found in Ovx rats treated with 17β estradiol alone. In contrast, boron in combination with PTH did not provide an additional benefit for Ca, P, and Mg balance or bone quality when compared with PTH alone, but significantly improved the percentages of apparent Ca absorption and retention compared with Ovx rats without treatment. The improvement of Ca balance by the combined treatment with PTH plus boron was not found in Ovx rats receiving PTH alone. In conclusion, 17β estradiol in combination with boron that was initiated 6 weeks after ovariectomy, provided a more beneficial regimen than boron or 17β estradiol alone for postmenopausal bone loss in Ovx rats. The importance of the present findings is that boron has the potential to improve estrogen action on bone and Ca balance, in particular when estrogen is initiated at the late state following menopause.