Fourteen patients with acute exacerbations of chronic bronchitis and hypercapnia received two treatment periods with Intermittent Positive Pressure Breathing, the ventilator being driven by gas containing about 24% or about 45% oxygen. Arterial PO2 and PCO2 were measured before, during and after each treatment. The results demonstrated that increasing hypercapnia did not, as a rule, occur when 45% oxygen was used as the driving gas. When hypercapnia did occur it appeared to be independent of the inspired oxygen concentration. The importance of short treatment periods, correct ventilator settings and supervision of the patient during and after treatment is emphasised.