Osteoporosis, characterized by reduction in bone density, is a significant source of mortality among the elderly, particularly in oestrogen-deficient women. We studied the effect of Withania somnifera (WS) root extract (ethanolic), which contains oestrogen-like withanolides for anti-osteoporotic activity. Female Sprague-Dawley rats were either sham operated (n = 12) or ovariectomized (n = 12) and treated with WS/vehicle (65 mg kg(-1)), orally for 16 weeks (n = 12). All rats were allowed free access to a calcium-deficient diet (0.04% Ca) and distilled water. At termination, urinary excretion of calcium (Ca) and phosphorus (P) and serum levels of Ca, P and alkaline phosphatase (ALP) were measured. Femur and tibia bones were processed for histological (histology), morphological (scanning electron microscopy, SEM), biomechanical strength (impact test) and mineral composition (ash) analysis. Ovariectomized (OVX) rats showed a significant increase in serum ALP levels and urinary Ca and P excretion. Histological findings revealed narrowed, and disappearance of, trabeculae with widened medullary spaces in the OVX group. Ash analysis showed a reduction in ash weight, percent ash, ash Ca, ash P and ash magnesium levels in the OVX group. Further, SEM examination revealed metaphyseal bone loss in femurs and impact test showed a reduction in biomechanical strength of tibias in OVX rats. WS treatment markedly prevented the above changes in OVX rats and thus may be a potential agent in the treatment of osteoporosis.