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Summary. Electron microscopic evidence is presented to indicate that entire megakaryocytes traverse the marrow-blood barrier and enter the circulation. Passage occurs through apertures of 6 μm in diameter, located in the parajunctional areas of the marrow sinus endothelium. Serial sectioning indicates that these apertures are transendothelial rather than interendothelial. The cytoplasm of these megakaryocytes form sinuating elongated projections which may release their platelets in the sinus lumen or when reaching the pulmonary circulation. In the extravascular compartments, megakaryocytes are preferentially located in the subendothelial region. In this location they can send numerous organelle-free projections into the lumen. These projections distinguishable from less numerous organelle-containing projections could serve to ‘anchor’ the cell to the endothelium. They could also serve to ‘monitor’ the circulation and to receive information as to the requirement of body for platelet formation.