Bone in vivo stresses and moments were determined from rosette strain recordings obtained from the mid-diaphysis of growing exercising rats. Two activity groups were examined beginning at 3 weeks of age: 2 min day-1 and 45 min day-1 at 0.2 ms-1 in an exercise wheel. In vitro moment-strain curves were obtained during mechanical calibration tests on intact femora, and area inertial properties were determined from the mid-diaphysis cross-sections. The mechanical calibration and histomorphometry procedures were then used to compute functional stresses and moments based on the in vivo rosette strain recordings. During the period 6-30 weeks of age the rats increased in body weight over threefold, but no significant changes in principal strain and stress magnitude or orientation were found. Peak in vivo compressive and tensile moments increased during growth in proportion to the animal mass squared, but the ratio of these moments to animal body weight times bone length (BWBL) remained constant throughout growth and in the adult. The parameter BWBL appears, therefore, to be a useful predictor of long bone functional strength. Peak torsional moments remained a constant 8.1 +/- 3.0% of the ultimate torsional strength, providing a safety factor of approximately 12. Differences in the in vivo moments between the two activity groups were found, which were due primarily to adaptive, but not significant, changes in bone geometry. These findings support the hypothesis that long bones model and model during growth and altered activity in order to regulate the functional strains at a predefined level.