The relationship between acid base status and mineral metabolism after heavy exercise has been examined in 12 healthy subjects. Following burst exercise (duration 60-130 sec) to the point of exhaustion, blood pH had decreased (7.42 +/- 0.01 vs. 7.18 +/- 0.02, P less than 0.001) and plasma ionized calcium had increased (1.09 +/- 0.01 vs. 1.22 +/- 0.02 mmol/liter, P less than 0.001). Log ionized calcium concentration showed a significant negative correlation with pH (r = -0.90). Although plasma total calcium increased after exercise (2.47 +/- 0.05 vs. 2.67 +/- 0.04 mmol/liter, P less than 0.001), this change was not seen if the observed values were corrected for the accompanying increase in plasma protein concentration, suggesting that hemoconcentration accounted for these increments. Significant increases were also seen in plasma inorganic phosphate concentration, though not in plasma magnesium. Radioimmunoassay of parathyroid hormone using two different region-specific assays, one directed at the mid-region/carboxy-terminal and the other at the amino-terminal portion of the molecule, and of calcitonin, showed no change during exercise-induced hypercalcemia. The results do not suggest significant skeletal buffering of this type of acidosis and indicate that the changes in ionized calcium associated with short bursts of intense exercise are directly related to acidosis and that those in total calcium are a consequence of hemoconcentration.