LaBrie, Paladino, Bronzino, and Thrall automate in-vivo measurement of quasi-static lung compliance in the laboratory rat. After stopping ventilation, a balloon valve seals off air flow. With a transducer to measure air and esophageal pressure, we inflate the lung to maximum volume by opening a valve to air under pressure. Measurement of quasi-static lung compliance on deflation is more indicative of lung injury than static lung compliance or inflation elasticity.