1. The rate of utilization of blood platelets in radiated, thrombopenic cats has been measured directly. Thrombopenic animals, incapable of significant platelet regeneration, were cross circulated via carotid to carotid anastomoses with normal animals. After return to independent circulation the rate of disappearance of cross circulated platelets was measured by periodic counts.
2. By this method it was possible to elevate the platelet count anywhere from 100,000 to around 400,000 per cubic millimeter. The highest count obtained followed cross circulation with a splenectomized animal. In most instances the platelet level attained was within the physiologic range of that found for normal cats by the same method.
3. The cross circulated platelets gradually disappeared from the circulation over a two to a slightly more than four day period. Under the conditions of this experiment the entire platelet mass would have to be replaced therefore every two to five days. The same figures probably apply within narrow limits to the normal cat.
4. The average rate of platelet utilization was approximately 2500 per cubic millimeter per hour. In seven of eight experiments the rate of disappearance varied from about 1600 per cubic millimeter per hour to about 2800 per cubic millimeter per hour. In the experiment using a splenectomized donor the rate of disappearance was about double the average for the rest of the group.
5. Attention is called to possible therapeutic implications of these findings in idiopathic thrombopenic purpura.