Using the rapid infrared CO2 analyzer the alveolar CO2 of two major groups of patients with acute poliomyelitis were followed with almost daily observation from the day of admission for varying lengths of time. Group I consisted of 19 patients whose vital capacity drop or clinical condition did not warrant use of respiratory assistance. Of this group 13 (69%) maintained a normal alveolar CO2 of 35–45 mm Hg, 5 (26%) hyperventilated and 1 (5%) underventilated. Group II consisted of 35 patients who required tank respirator assistance. Before they were placed in the respirator 24 (69%) were maintaining a normal alveolar CO2, 6 (17%) were hyperventilating and 5 (14%) were underventilating. As soon as the patients were placed in the respirator, the alveolar CO2 began to drop so that by 24 hours the average was 25 mm Hg (ranging from 16 to 30). This level was maintained with minor variations for as long as the patients were in the respirators—some patients were followed for as long as five months. With clinical improvement and return of breathing capacity, some of these patients were allowed more and more time breathing on their own. The alveolar CO2 returned to normal gradually so that by the time the patient no longer needed respirator assistance it was within normal limits. Simultaneous arterial ph and alveolar CO2 determinations were made on 30 cases in which there was a possibility of metabolic acidosis. The ph ranged from 7.33 to 7.68 with a mean of 7.50. The tank respirator appeared to create a demand for excessive ventilation, which could be alleviated by using other equipment. Submitted on August 26, 1955