It has been showed that the degeneration and disappearance of pulmonary surfactant is one of the causes of post-perfusion lung syndrome which occurs after cardiopulmonary bypass. An experimental study was undertaken to reveal the change of the osmiophilic body (a precursor of pulmonary surfactant) during cardiopulmonary bypass. Dogs were divided in 5 groups: I Control group II Group with whole blood III. Group with 20 % hemodilution IV. Group with no blood V Group with 20 % hemodilution and CDP-choline Results The following results were obtained: 1) Increase of osmiophilic bodies as ratio of the blood was increased in priming solution. 2) Increase of osmiophilic bodies in groups II, III and IV as cardiopulmonary bypass time was prolonged. 3) Osmiophilic bodies was markedly reduced by adding CDP-choline.