Immobilization of load-bearing bones results in imbalance of bone turnover followed by bone loss and impairment of its mechanical function. The question is whether immobilization induced bone loss is accompanied by deterioration of properties of the bone tissue components. Thermally induced transformations of collagen reflect the overall condition of the structure and cross-links in collagen network. The aim of the present study was to investigate whether immobilization induced osteopenia effects stability of collagen in bone tissue. Bone loss was developed by unilateral hindlimb immobilization in adult rats. Effects of unloading on cortical tissue from tibiae were studied after three weeks of unloading (I3R0) and four weeks after remobilization and free convalescence (I3R4) in both tibiae. Thermodynamic parameters of collagen degradation in bone were determined from differential scanning calorimetry (DSC) analysis of partially dehydrated cortical bone samples from tibiae in the range of temperatures from 60 degrees C up to 300 degrees C. All bone samples were thermally very stable showing first clear endothermal process with a peak temperature within a range from 150 degrees C to 169 degrees C, for different samples. The next endotherm, wider and flatter, was observed between 245-298 degrees C with a peak at 255 degrees C - 260 degrees C. There were significant side-to-side (right to left) differences for both endothermal processes in tibiae samples from experimental groups: I3R0 and I3R4. Immobilization of load-bearing bones influences stability of collagen in bone tissue. Free remobilization was not sufficient for recovery of thermal parameters of bone.