In order to gain an understanding of local and humoral factors affecting bone maintenance, the effect of three weeks splintage of a single hind limb on the mid tibial metaphyseal area and mineral content of the splinted and the contralateral hind limbs was assessed immediately after splint removal and following one month of mobilisation in 12 week old Sprague Dawley rats. The breaking strain and elastic energy of both femurs was also determined following four weeks of mobilisation. All parameters were compared with those of unsplinted rats so as to evaluate local and general effects of splintage on growth and mineralization Immobilisation significantly reduced tibial metaphyseal bone mineral density (BMD) in the immobilised limbs compared to that in the 'free' limbs of splinted animals and controls. These changes persisted and were accentuated by relatively greater increases in the BMD of the limbs of the unsplinted (control) animals after seven weeks. Splintage equally reduced the tibial metaphyseal area of both the immobilised and the free limbs of the splinted animals compared to those of unsplinted animals. This effect was not apparent following four weeks mobilisation The total mineral content (BMC) of the tibial metaphyses was lower in both the immobilised and contralateral 'free' limbs of splinted animals. Whilst splintage did not change the relationship between mineralisation and area in the immobilised tibiae of splinted animals from that of unsplinted animals it changed this relationship in the free limbs of the splinted animals. The latter effect was no longer present four weeks after the removal of the splint although the extent of mineralization was impaired in the immobilised limb. Splintage reduced the breaking strain and the breaking energy of the mid shaft of both the immobilised and free the femurs of splinted animals, an effect that persisted for four weeks after the removal of the splint. This variation was correlated with an effect of splintage on femoral size with some additional local effect from immobilisation. Thus osteoporotic changes in the middle area of the tibial metaphyses during immobilisation, in a functional position, without compression, result from a combination of local effects on mineralisation and more general effects on growth, the latter influencing the elastic properties of femur shafts via change in the relative area of the shaft that is subject to biomechanical testing.