Summary. Rats were given exchange transfusions with different volumes of platelet-poor blood, injected 24 hr later with tritiated thymidine, and killed after another 24 or 36 hr. Autoradiograms of bone marrow smears were made and the labelling index of megakaryocytes was determined. The labelling index was proportional to the degree of thrombocytopenia induced by exchange transfusion. The results therefore demonstrate that the number of circulating platelets exerts a rather delicate control over megakaryocytopoiesis. In addition, the potential of this procedure for studying megakaryocytopoiesis, its regulation, and its abnormalities is indicated.