Quasi-static elastic properties and dynamic compliances were measured in a series of patients undergoing open-heart surgery with total cardiopulmonary bypass for periods up to 2 hours. The data revealed that in half of the patients no alterations occurred, and in the other half there appeared a distinct improvement in lung mechanics, as indicated by the elastic properties. The reasons for this improvement are not known but may be related in part to changes in pulmonary blood volume. Artificial perfusion per se for around 2 hours causes no significant impairment in the elastic component of lung mechanics.