Book excerpt: The pattern of recovery of the respiratory system from blast injury was investigated in sheep exposed to overpressures in a shock tube. Measurements of the pH and blood gas tensions, determinations of the venous- admixture (Qs/Q) and the alveolararterial oxygen gradient (A-a)O2 were conducted before and at intervals up to 132 days following injury. There was an immediate marked increase in Qs/Q, reduction in PaO2, and a moderate increase in (A-a)O2, with very little change in the pH or PCO2 of the arterial blood. The greatest recovery was evident within 24 hours with further gradual improvement seen 2, 7, 14, and 21 days after exposure. After the 21st day, most of the animals exhibited virtual complete recovery of the functional efficiency of the pulmonary system as tested at rest.