1. In heart-lung preparations perfused with heparinized blood, clumping of platelets and a sharp fall in the platelet count occurred in the first 5 to 15 minutes after the external circuit was established.
2. In 10 such preparations surviving from 1 to 4 hours, only one showed a return of the platelet count to the level existing before operation.
3. When platelets were reduced in numbers before the blood was used for perfusion, the count tended to level off after a small recovery, if any.
4. An estimate of clumping showed it to bear an inverse relationship to the platelet recovery.
5. No satisfactory evidence of platelet production in the lungs has been obtained in these studies with heart-lung preparations.
6. The possibility of special factors which may influence platelet formation is discussed.