Various mammalian species such as rats, dogs, sheep, pigs, rabbits, guinea pigs and monkeys have been used to model osteoporosis. Osteoporosis has been induced by ovariectomy (OVX), immobilization or space flight, low calcium diet, orchidectomy. In contrast to OVX rats, orchidectomized rats have decreased body weight gain and increased cortical porosity. Cancellous bone osteopenia following orchidectomy appears to be due to decreased growth and a reduced rate of bone turnover, with resorption exceeding formation. OVX rats have been extensively used to study peri- and post-menopausal skeletal changes in humans. Dual X-ray absorptiometry can detect accelerated trabecular bone loss in the region of the distal femoral metaphysis, in the femoral neck and in the lumbar spine in OVX rats. OVX induces short-term high-turnover accelerated deterioration of three-dimensional trabecular structure in mice. Magnetic resonance imaging showed OVX-induced losses in trabecular bone volume fraction and structure that oestrogen replacement therapy prevented.