The purpose of this study was to examine the effect of a recovery period between two periods of exercise on bone metabolism and related hormones. Concentrations of serum parathyroid hormone ([PTH]), plasma ionized calcium ([Ca(2+)]) and total calcium were measured. A group of 12 healthy men aged 20-27 years participated in this study. They took part in two exercise protocols (P(1) and P(2)) on two separate weeks. The exercise in P(1) comprised two successive periods of 21 min each at 70% and 85% of maximal oxygen uptake; P(2) comprised two periods of exercise at the same intensities but separated by 40 min of recovery. Venous blood samples were collected 1 day before the sessions (control), before each protocol, during (7th and 21st min), at the end (42nd min in P(1) and 82nd min in P(2)) and after 24 h of recovery. The [PTH] was significantly elevated during the two protocols ( P<0.01), remained raised in P(1) after 24 h of recovery ( P<0.05) and was significantly lower ( P<0.01) at the end of P(2) when compared to P(1). The [Ca(2+)] decreased significantly during and at the end of the two protocols ( P<0.01) and had returned to control values after 24 h of recovery. Plasma lactate concentration increased during the two protocols ( P<0.01) and returned to control values after recovery. These results indicate firstly that [Ca(2+)] decreases during continuous exercise as [PTH] increases and remains raised after 24 h of recovery, secondly that a recovery period between two periods of exercise attenuates the variations in [Ca(2+)] and [PTH], and thirdly that recovery may have anabolic effects on bone. However, the small physiological changes observed prevent us from forming any firm conclusion about this.