The measurement of carbon dioxide tension (Pco2) owes its development to the 1952 polio epidemics in Copenhagen and the United States, during which artificial ventilation was first widely and effectively used and it was necessary to assess its effectiveness. Pco2 had been determined by various "bubble methods" in which carbon dioxide (CO2) was measured in gas equilibrated with blood at body temperature, or by one of two methods using the manometric apparatus of Van Slyke: interpolation on a plot of CO2 content versus equilibration gas Pco2 or use of the Henderson-Hasselbalch equation to calculate Pco2 from pH and plasma CO2 content. In 1954 Richard Stow described a CO2 electrode--a new concept--using a rubber membrane permeable to CO2 to separate a wet pH and reference electrode from the blood sample. This was the first membrane electrode, a device now used in hundreds of different ways. Severinghaus developed Stow's electrode, stabilizing it with a bicarbonate-salt solution and a spacer. The CO2 electrode concept had occurred to Gesell in 1925, but for measurement of gas only, and to Gertz and Loeschcke, who were unaware of the Stow-Severinghaus electrode, in 1958. The development of the CO2 electrode terminated the use of bubble methods, the Van Slyke methods, and the Astrup technique and at the same time reinforced the Astrup-Siggaard-Andersen acid-base analytic theory.