Ovariectomized rats have been used extensively and have received substantial acceptance as animal models for postmenopausal osteoporosis. However, little is known about the microscopic tissue properties of rat vertebral bone, especially during osteoporosis caused by estrogen depletion. This study applied a new nanoindentation technique to quantify the microscopic mechanical properties of vertebral trabecular bone tissue in ovariectomized rats. Seventeen-week-old Sprague-Dawley rats underwent an ovariectomy. After death at 37 weeks, the fraction of the trabecular bone area of the lumbar vertebrae (L4) was measured with scanning electron microscopy and the elastic modulus and hardness were determined with the nanoindentation technique. The bone area fraction was reduced significantly after the ovariectomy. However, the elastic modulus and hardness did not change significantly at the microscopic level. The results indicate that estrogen-dependent osteoporosis in rats manifests in a loss of bone mass whereas the elastic and hardness properties of the surviving bone tissue remain relatively unchanged.