After drawing a large amount of blood (15 cc/kg) once from guinea pigs and observing changes in the platelet count. the author recognized ,in increase in the platelet count up to the double the normal count 2-6 hours after the blood drawing; and also 10-14 th day after the depletion an intermediate degree of the increase in platelets was observed. Therefore, by performing bone-marrow tissue culture taken from the femur of animals at the time when there was an increase in the platelet count, the author studied the megakaryocyte function and obtained the following results: 1. No change in the function and megakaryocyte count at all could be observed 2-6 hours after drawing blood when there was an increase in the platelet count. 2. On the 10-14 th day after blood-depletion at the time when there was an intermediate increase in platelets the megakaryocyte function was markedly accelerated and also the megakaryocytecount was increased to about double the normal count. Summarizing the above, the increase in the platelet count within 24 hours after the drawing of a large amount of blood is caused simply by the mobilization of platelets towards the peripheral blood; and the sccondary increase in the platelet count on the 10-14 th day is induced by the iucrease in the megakaryocyte count and function.