Bone CO2-titration curves were obtained in mature rats weighing 500-600 g. Animals were exposed for one hour to 1, 3, 5, 10, or 15% CO2 in air. Measurements of bone CO2, were made using a modified titrimetric analysis on fresh and oven-dried samples of paired rat femurs. A manometric method was used for comparison. Arterial blood samples were obtained for measurements of partial pressure of CO2 in arterial blood (PaCO2). Within the range of environmental CO2 concentrations studied, a linear relationship was observed between the PaCO2 and the increment in fresh bone CO2 content. This relationship is defined by the equation: delta fresh bone CO2 (mmol/kg) = 61.8 +/- 0.68 PaCO2. The CO2 increment was confined to a heat-labile, presumably soluble pool comprising 10.5% of the total bone CO2 content. No change in the water content of the bone was observed as a result of acute CO2 exposure. The results of this study demonstrate the rapid in vivo CO2 uptake of bone in response to exposure to increased CO2 levels.