Physical activity has been suggested as one of the determinants of bone turnover and to prevent the involutional age related bone loss. However, the degree to which physical exercise is necessary to induce changes in bone turnover and calciotropic hormones have been widely discussed (Williams et al., 1984; Cook et al., 1987; Smith et al., 1985). The aim of this study was to examine the rate of bone formation measured by osteocalcin in 56 healthy volunteers before and after 4 and 8 weeks of physical exercise (PE) and its dependence on various parameters of calcium and phosphate metabolism. The studied group consisting of 44 men and 12 women, mean age 24.8 and 24.3 years, respectively, performed a standardized physical training of 8 weeks. Mean serum osteocalcin levels were significantly (p < 0.01) reduced after 4 weeks (men: 2.26 ± 1.8 ng/ml; women: 0.94 ± 1.6 ng/ml) compared to the values before PE (men: 4.01 ± 2.18 ng/ml; women: 1.69 ± 1.7 ng/ml) and returned to normal values after 8 weeks. Similiarly, magnesium levels (0.82 mmol/1) decreased significantly (p < 0.01) after 4 weeks of PE (0.79 mmol/1), returning to normal values after 8 weeks. Concomitantly, there was only a slight, but significant fall of serum calcium from 2.48 ± 0.07 to 2.45 ± 0.07 returning to initial values again. Furthermore, serum phosphate increased slightly in men from 1.01 mmol/1 to 1.13 and 1.15 mmol/1 after 4 and 8 weeks, respectively. In contrast, alkaline phosphatase and serum creatinine remained in the normal range. Serum PTH values (men: 222 pg/ml; women: 223 pg/ml) increased slightly after 4 weeks of PE (men: 234 pg/ml; women: 250 pg/ml; p < 0.1) and decreased after 8 weeks (men: 186 pg/ml; women: 177 pg/ml). It seems that the first period of adaptation to physical activity results in a temporary fall of calcium, magnesium and osteocalcin levels with a reactive increase of PTH. It is conceivable that the decrease of osteocalcin levels is due to an increase of stress hormones, especially glucocorticoids being well known to decrease bone formation. After a further 4 weeks period of PE most values return to initial values by adaptation to physical activity. In contrast to AP, osteocalcin seems to be a sensitive parameter for changes of bone formation by PE even, if the time studied is short.