Like erythro- and granulopoiesis megakaryocytopoiesis (MK poiesis) is controlled by specific feed-back mechanisms. Platelet production has been found to be regulated predominantly by the number or mass of circulating platelets. Our present understanding of MK poiesis is based on data from 3 major experimental areas, which are individually discussed in this paper. In vivo experiments on thrombocytotic or thrombocytopenic mice. Radioisotopic assays for thrombopoietin determination. Investigation by in vitro culture methods of megakaryocyte colony-forming precursor cells (CFU-M).