A total of 17 patients with hypertension undergoing renal vein or adrenal vein catheterization were investigated in order to ascertain the number of megakaryocytes in blood from the inferior vena cava, the femoral artery and a cubital vein. On an average 11.9, 3.8, and 4.5 megakaryocytes per ml were found, respectively. In blood from the inferior vena cava, 30% of the megakaryocytes had copious cytoplasm, while megakaryocytes in arterial and cubital venous blood had sparse or no visible cytoplasm. It was demonstrated that 2/3 of the megakaryocytes were retained in the pulmonary circulation and that at least 70% of the platelets could derive from megakaryocytes in central venous blood or the pulmonary circulation. It was found that megakaryocytes pass through a life cycle in which the differentiation take place in the bone marrow, platelet release occurs mainly in central venous blood and in the pulmonary circulation and the destruction of the megakaryocyte nucleus take place outside the bone marrow, especially in the pulmonary circulation.