Normothermic autoperfused heart-lung preparation has the advantages of avoiding ischemic time and allowing continuous monitoring of organ function during preservation. When this technique is used, the lungs deteriorate quickly, but the reasons for this deterioration have not been investigated. This study was designed to explore the possible cause of rapid lung deterioration. Three groups of mongrel dogs were used. In Group 1 (N = 5), a buffer bag was used in the heart-lung preparation. In Group 2 (N = 6), a 20 mu filter was incorporated between the buffer bag and the right atrium. In Group 3 (N = 5), no buffer bag was used. Average survival time was 15.0 +/- 3.1 hours in Group 1, 13.5 +/- 0.7 hours in Group 2, and 21.6 +/- 2.3 hours in Group 3. Heart function was comparable among the three groups, but the arterial pulse pressure was lower and the heart rate higher in Group 3. Both white blood cell and platelet counts decreased contonuously during the preservation period. Examination of the filters in Group 2 revealed numerous aggregates consisting of platelets, white blood cells, red blood cells, and fibrin. Small thrombi were also found in the lungs in Groups 1 and 2. The results indicated that one important reason for quick lung deterioration was numerous aggregates, which were formed in the buffer bag, returned from the venous line, and trapped in the lungs. Removal of the buffer bag reduced the production of aggregates but tended to de-stabilize the hemodynamics of the preparation.