This paper is based on the experience gained during the 1952 epidemic of poliomyelitis in Copenhagen, in which there were many more severe cases than in any previous epidemic in Denmark. The management of this epidemic is described by Lassen. We would like to stress that the ideas and the work behind this presentation result from close co-operation and numerous daily discussions between all the physicians involved in spells of duty of 12-16 hours a day for months, ably and enthusiastically assisted by the nurses and medical students.Our tasks in the laboratory were (1) to control the effectiveness of ventilation; (2) to control the circulatory collapse, the renal function, the fluid balance, etc.; and (3) to assist in evaluating the ventilatory effect and the influence on circulation of the different types of mechanical respirator. Control of VentilationIn the control of gas exchange, the pH, the C02 tension in arterial or in venous blood, and the oxygen saturation of arterial blood were determined.The CO2 tension was calculated from the blood pH and the total plasma C02 according to the following two formulae: pH= 61 +lgconc. H,COj total CO, =HCOj-+H,CO, In the first, which is that of Henderson-Hasselbach, the numerator is the CO2 carried as bicarbonate, while the denominator is the amount in physical solution. We estimate the t6tal CO2 content of plasma by Van Slyke's In several patients we found, within a short period of time, colossal changes in pH, in the total carbon dioxide in plasma, and it the C02 tension.In a 5-year-old boy, almost moribund on admission, the pH of venous blood was 6.99, the total CO2 in plasma 39 mMol and the CO2 tension 150 mm. Hg. After tracheotomy and manual artificial ventilation the pH had increased in two hours to 7.52 and fifty-five minutes later to 7.65, accompanied by a fall in the total CO2 content in plasma as well as in the CO2 tension (Table IT).