Seven patients in acute exacerbation of chronic respiratory failure were given 24.5% and later 28% oxygen through Ventimasks. The mean increases in arterial PO(2) were 11 and 21 mm. Hg while breathing 24.5% and 28% oxygen respectively compared with control values while breathing air. Associated increases in arterial PCO(2) were 4 and 8 mm. Hg, respectively. In five of the patients these increases in inspired oxygen concentration resulted in useful increases in tissue oxygen supply without significant deterioration in ventilation, but in two patients arterial PCO(2) rose excessively and artificial ventilation was required.