Short daily periods of controlled dynamic loading were applied in vivo through the flexed carpus and olecranon to the intact ulna of 240 g male Sprague-Dawley rats. This technique involved neither surgical preparation, nor direct loading of the periosteum at a site close to the region of the bone in which adaptive modeling was subsequently assessed. The animals used their limbs normally between loading episodes, thus approximating to the natural situation, in which short periods of exercise are generally superimposed on longer periods of less strenuous activity. The strain patterns associated with normal activities were established for the rat ulna from strain gauges implanted in vivo. Typical peak strain magnitudes during unrestricted locomotion varied between -0.0007 and -0.0012, with peak strain rates between 0.023 and -0.038 sec-1. Stride frequency was 1.5-4.2 Hz. The adaptive response to a single 10 min period of loading each day, causing peak dynamic strains of -0.002 (1200 cycles at 2 Hz, and a loading/unloading rate of +/-0.03 sec-1), involved modification of the normal growth related medial to lateral modeling drift, simultaneously reducing the rate of lateral periosteal bone deposition and medial bone resorption. This change to the normal modeling pattern reduced the total amount of new bone formation as well as the midshaft curvature of the ulna. At higher peak strain amplitudes (-0.004), adaptive straightening was accompanied by an increase in bone mass, achieved by an increase in the mineral apposition rate on the previously forming lateral face, and arrest of resorption on the medial ulna surface, with reversal to formation. These experiments show that the growing rat ulna underwent adaptive changes in both bone mass and architecture when short daily periods of axial loading, producing strains within the physiological range and with near normal strain distribution, were superimposed on the loading associated with normal activity. At moderate peak strain magnitude (-0.002), modification of drift produced a straighter bone, associated with a reduced periosteal bone formation. At higher strain magnitude (-0.004), adaptive modeling produced a straighter bone associated with increased periosteal bone formation.