The limbs of growing chicks (2-12 wk of age) were subjected to differing conditions of mechanical use to examine the effect of extrinsic loading on bone modeling early in postnatal growth. One group of chicks was subjected to intensive exercise by running on a treadmill 5 days/wk at 60% maximum speed while carrying on their trunk a load equal to 20% of body weight (EXER). In a second group, weight-bearing function was eliminated by sciatic denervation of one hindlimb at 2 wk of age (DNV). A third group grew under sedentary conditions (SED). Comparisons among groups were made on the basis of bone mass and length, cortical cross-sectional area and second moment of area, cortical thickness, longitudinal curvature, and % ash. After normalizing for growth-related differences in body mass among the three groups, we found that exercise led to an overall 16 +/- 13% increase in cortical cross-sectional area and a 26 +/- 21% increase in second moment of area measured at proximal, midshaft, and distal levels of the bone compared with values of SED animals. These increases in cortical geometry corresponded to a 10% increase in total bone mass and were generally established by 8 wk of age (6 wk of training) and maintained to 12 wk of age. When deprived of functional use, the growing bones of DNV animals were reduced in mass (-19%), cortical area (-8 +/- 7%), and second moment of area (-11 +/- 9%) compared with SED animals. DNV tibiotarsi were also significantly shorter (7% at 8 wk and 14% at 12 wk); however, the contralateral load-bearing tibiotarsus of the DNV animals was similarly reduced compared with SED and EXER animals, suggesting a general reduction of growth in the DNV group. Even more pronounced than the reductions in bone mass and area, however, were the loss of normal longitudinal curvature and an increase in the variability of cross-sectional shape and cortical thickness of the DNV tibiotarsi compared with SED and EXER animals.