1. Carbon dioxide titration curves were determined in vivo in dog and man at various degrees of acute non-respiratory acidaemia and alkalaemia.
2. The slope of the CO2 titration curve (Δlog Pco2/ΔpH) was found to increase with the severity of the acute non-respiratory acidaemia. In states of acute non-respiratory alkalaemia the slope (Δlog Pco2/ΔpH) tended towards unity.
3. A simple scheme based on the CO2 titration curves determined in vivo has been proposed for the assessment of acute acid-base disturbances in man.
4. Carbon dioxide titration curves were also determined in vivo in patients with chronic respiratory and non-respiratory acidaemia and it was found that these curves were not significantly different from those obtained in states of acute acid-base disturbances. It is therefore suggested that the scheme described in this paper is applicable to all acid-base disturbances.