Guinea pigs were exposed for up to 8 weeks to 0.5% CO2, 21% O2, and balance N2. Control groups of the same age were kept simultaneously in environmental chambers on air. A slight increase in PaCO2 and decrease in pH were observed at various periods of exposure to 0.5% CO2. After eight weeks of exposure, an increased kidney calcification, indicated by increased kidney calcium content, was found. Plasma calcium was significantly elevated at this point, apparently because of the release of calcium from bone. After 8 weeks of recovery on air that followed 8 weeks of exposure to 0.5% CO2, values had returned to control levels. No significant ultrastructural changes were found in the lungs after 4, 6, and 8 weeks of exposure to 0.5% CO2.