The aim of this controlled study is to investigate the effect of heparin on osteoporosis initiation and of calcitonin and tamoxifen on the progress of osteoporosis induced by heparin through biomechanical means and, thus, to assist in clinical usage of these drugs. 32 four-month-old female New Zealand white rabbits were divided into four different experimental groups. The animals in group A were administered heparin (Liquemine) intraperitoneally at the dosage of 1000 IU/kg/day. The animals in group B were injected the same of heparin as those of group A, and in addition, were given calcitonin at the dosage of 100 IU/kg/day. The animals in group C were medicated the same way as group B but 2 mg/kg/day tamoxifen (Nolvadex) was orally added into their intestine via cannula, one side connected to the injector. The animals in group D were the control. The experiment lasted 8 weeks. The animals in all experimental groups showed the same growth patterns as that of the control group. Whole-bone femur, humerus and tibia specimens were subjected to 3- point bending tests while sections from the proximal ends of the same specimens were subjected to compression tests. The data, recorded as load vs deflection, were converted into stress vs strain using the strength of materials formulae. The data obtained from the bending and the compression experiments were treated separately. The stiffness of the bones of the medicated groups were compared with those of the control groups. Our data indicated that the tamoxifen treated humera, femora and tibiae attained the largest bending stiffness in all cases investigated. However, this was not the case for compression. None of the drug administered groups attained the stiffness of the control group except for the case of tamoxifen treated femora which attained stiffness close to that of the controls. The results show that heparin altered the mechanical properties of bones indicating osteoporosis, tamoxifen was effective in reducing the effect of heparin while calcitonin yielded no conclusive result.