We investigated the relationships among bone mass, bone cell activities, and exercise level in 20 healthy 61–77 year old male volunteers divided into three groups according to the time they physically trained per week: nine subjects training less than 3 hr/week, five subjects between 3 and 6 hr/week and six subjects more than 6 hr/week. Physical performance was evaluated by Vo2 max (ml min−1 kg−1). After tetracyline double labeling, iliac crest biopsy was obtained from each subject. The longer the physical activity, the higher the Vo2 max. Subjects exercising between 3 and 6 hr/week revealed higher adjusted appositional and bone formation rates than all the others; mass and structural parameters also showed higher (nonsignificant) values. For the whole population V̇o2 max appeared negatively related to cortical thickness, cancellous bone volume, and trabecular thickness. These alterations were accompanied by increased cancellous bone turnover; this was evidenced by an increase in activation frequency and in resorption and formation rates as V̇o2 max increased. The bone remodeling periods tended to decrease also. Whatever the bone turnover rate, subjects were in steady state as far as their bone balance was concerned. Relationships between V̇o2 max and mineral apposition rate on the one hand and V̇o2 max and resorption surface on the other hand were best fitted by a quadratic model, suggesting a possible nonlinear effect of physical training on bone mass. We hypothesize that there is a threshold (6 hr/week) determining different effects. Adjustment of bone mass and trabecular arrangement were completed at time of biopsy and reflected probably past and transient bone imbalance. Other studies are needed to corroborate this assumption. © 1993 Wiley-Liss, Inc.