Arterial and venous blood samples were obtained from 23 patients with acute asthma before they received systemic or intermittent positive-pressure breathing therapy. Serum lactate, electrolyte, and enzyme concentrations, arterial pH, arterial PCO2, arterial PO2, peak expiratory flows, and 2,3-diphosphoglycerate concentrations were measured. The serum glutamic oxaloacetic transaminase, lactic dehydrogenase, and creatine phosphokinase activities were increased in 90, 81, and 38 per cent of the patients, respectively, and the individual values correlated poorly with the arterial PO2. In addition, there was a close correlation between the 2,3-diphosphoglycerate concentration and arterial PO2 (r = -0.75), indicating an active intraerythrocytic compensatory mechanism for enhancing O2 delivery operative in acute asthma.