Understanding the reaction of bone to physical exercise is important for the development of strategies to increase and maintain bone mass. In this study the aim was to investigate the relationship among exercise intensity, physical capacity, and the biochemical responses, estimated by measuring biochemical markers of bone metabolism in serum. As a complement to the circulating concentrations we also accounted for the plasma volume shifts during and after exercise. The study included 10 men and 10 women, mean age 29 years, with a wide range of physical capacity, who performed a standardized running exercise test on a motor-driven treadmill with loads corresponding to 47 and 76% of VO2 max (maximal oxygen uptake) followed by a maximal effort until exhaustion. Total work time was about 35 minutes. Venous blood samples were drawn at rest, after each load, and after 30 minutes and 24 hours of recovery. The reductions in plasma volume during exercise were 4.3% (P < 0.05) and 15.1% (P < 0.001) whereas after 24 hours in recovery there was an expansion of 7.5% (P < 0.001). There were marked, intensity-related, increases of PICP and tALP concentrations (P < 0.001) during exercise. Since these were of the order of plasma volume reduction they did not correspond to a change in the calculated circulating amount (content). However, as the concentrations returned to basal during recovery, the total circulating amounts were increased at this point (P < 0.05). Osteocalcin was also increased during recovery (P < 0.01), although concentrations were unchanged during the entire study. The amount (P < 0.001) and concentration (P < 0. 05) of ICTP were also increased during follow-up. Serum PTH concentrations rose (P < 0.05) in proportion to the intensity of exercise and remained elevated during recovery. The subjects' VO2 max demonstrated positive relationships to the biochemical responses to exercise in bone and BMD of the legs, and a negative relationship to basal PTH levels. Bone turnover and PTH secretion was stimulated by exercise, and low basal levels of PTH and high BMD were induced by a high level of physical fitness. These observations correlate well with the favorable effects of exercise and training on bone mass.