Torsion testing is considered a suitable method for measuring the strength of long bones. The aim of the current study was to investigate the effect of immobilization on the mechanical strength of rat bone and its relation to mineral content. The influence of freezing on the bone strength was also studied. The precision of the method, studied by comparing the maximum torque capacity of the contra-lateral tibiae of intact rats, was found to be 6.4%. No significant dominance of either side in the strength of the tibia could be observed. Tibiae that had been frozen for one week before thawing had a significantly lower maximum torque capacity and rigidity than fresh contralateral bones, the differences being −6.3 ± 3.2% (mean ± SEM) (p < 0.05) and −9.37 ± 9.48% (p < 0.05), respectively. The change in the maximum angle of deformation was not statistically significant. Three weeks' cast immobilization reduced the femur ash weight by 13.8 ± 2.5% (p < 0.001) relative to that of the non-treated contralateral leg. The maximum torque capacity and rigidity of the tibia also decreased in the immobilized leg, the differences being 11.6 ± 3.5% (p < 0.01) and 16.6 ± 5.3% (p < 0.01), respectively. The maximum angle of deformation was not significantly influenced by immobilization. The maximum torque capacity of the tibia correlated well with femur ash weight (r = 0.71, p < 0.001). The method used here is valid for measuring the strength of diaphyseal bones; it showed weakening of the bone during three weeks' cast immobilization.