This study has been conducted to examine the effects of physical exercise on the bone trabecular network and the cellular adaptations in three different areas of a single bone, the tibia. Male Wistar rats (9 weeks old) were treadmill-trained for 0, 3, 4, or 5 weeks at 60% of their measured maximal O2 consumption (VO2max). Histomorphometric analysis of the proximal tibia of running and age-matched control groups was performed in the epiphyseal trabecular bone, in the primary spongiosa and in the secondary spongiosa. Dynamic and static bone cell activities and serum calcium and phosphorus levels were measured. VO2max increased significantly by 18.4% after 5 weeks of training. In the epiphysis, a 9% increase in bone volume, associated with more numerous trabeculae (8%) was detected the third week of training. In primary spongiosa a significant increase (6.7%) in newly formed trabeculae was found. In secondary spongiosa bone volume increased significantly by 26.2% the fifth week of exercise and was associated with thicker trabeculae. The number of osteoclast profiles was significantly depressed. Osteoid surfaces and bone formation rate increased significantly in weeks 3 to 5. Serum calcium levels were found to be significantly decreased in weeks 3 and 4. There was no change in osteoid thickness or mineral apposition rate. These results suggest 1) a rapid increase in osteoblastic recruitment without change of the cell activity in response to moderate exercise; 2) a decreased bone resorption associated with a marked increase in bone formation from the third week of training; 3) adaptation of the trabecular network to exercise that seems to be bone-site-dependent, suggesting a cell sensitivity to training-engendered strain distribution within the bone or to strain-related local factors.