1. Fifteen healthy young men were exposed to an atmosphere of 0–7% CO2 in air for 7 weeks.
2. For 1 week before and during the exposure to CO2, serum calcium, magnesium and inorganic phosphate (Pi) levels were measured at intervals, together with 24 h urinary excretion of these three components, and net urinary acid excretion.
3. The urinary excretion of Ca2+, Mg2+ and Pi fell during exposure to CO2, whereas the serum levels of these components increased.
4. Serum levels of calcium were inversely correlated with urinary Ca2+ excretion both in control conditions and during exposure to CO2
5. An increase in urinary acid excretion in the third to fourth week of exposure to CO2 was associated with an increase in the urinary excretion of Ca2+, but not of Mg2+.
6. It is suggested that there is a difference from the normal in the renal handling of H+ and Ca2+ ions during exposure to CO2 which may lead to a retention of Ca2+ in the tissues.