Thrombocytopenia was produced in rats by exchange transfusion with platelet-poor homologous blood or heterologous antiserum. The subsequent rapid rise in platelet counts to thrombocytotic levels suggested accelerated platelet production. During this recovery period macromegakaryocytes were present in the bone marrow. The findings suggested that thrombocytopenia affected megakaryocytopoiesis at the precursor cell level with resultant macrocytosis.