It is the purpose of this presentation to review the unique structure and function of bone marrow anchored hematopoiesis in their significance for its response mechanisms to an exposure to ionizing radiation. The ultimate objective of bone marrow hematopoiesis is to maintain in the peripheral blood a constant level of the different blood cell types (erythrocytes, granulocytes, platelets, lymphocytes, etc.). All of them have their particular turnover kinetics (such as granulocytes 120 x 10(9)/d, erythrocytes 200 x 10(9)/d or thrombocytes 150 x 10(9)/d), are semi-autonomous in their steady state regulatory mechanisms and dependent on a life-long supply of mature cells from a stem cell pool with unlimited replicative and pluripotent differentiative potential. The present knowledge of hematopoietic cellular renewal is the result of years of basic experimental and clinical studies using radionuclides in various metabolic forms including (59)Fe, (32)P (DF (32)P), (51)Cr, (131)I, (60)Co, (3)H ((3)HTdR) and (14)C ((14)CTdR). To understand the physiology but in particular the radiation-pathophysiology, it is essential to recognize in detail the infrastructure of the bone marrow as a distinct unit. Indispensable for a life-long cell production is the capsule of the marrow - the bone cortex -, the arterial supply of blood connected to the sinusoidal microvascular architecture with its sinusoids contorti and recti as well as the central (cell collecting) sinusoids. It is further of importance to recognize the significance of nerval regulation of blood flow, characterized by myelinated and unmyelinated nerve fibers. The type of unique lining cells of the sinusoids is the prerequisite for the cell traffic between the hemopoietic parenchyma and the blood. This in turn cannot be achieved without an alternative opening and closing of the sinusoidal segments which - in turn - requires a rigid long capsule to assure an - in toto - constant volume of each bone marrow unit. If a bone marrow unit is exposed to ionizing radiation, a perturbance of the balance between cellular growth pressure and blood flow dynamics can be observed, resulting in a special type of bone marrow hemorrhage and an "excess cell loss" that may result in an non-thrombopenic exhaustion of the stem cell pool. Of great importance is the question as to the mechanisms that allow the bone marrow hemopoiesis to act as one cell renewal system although the bone marrow units are distributed throughout more than 100 bone marrow areas or units in the skeleton. The observation that "the bone marrow" acts and reacts as "one organ" is due to the regulatory mechanisms: the humeral factors (such as erythropoietins, granulopoietins, thrombopoietins etc.), the nerval factors (central nervous regulation) and cellular factors (continuous migration of stem cells through the blood to assure a sufficient stem cell pool size in each bone marrow "sub-unit"). It should be recalled that the bone marrow functions as a physiological chimera and becomes established by the hematogeneic seeding of stem cells to a mesenchymal matrix during embryogenesis. The repopulation of the bone marrow after partial body irradiation, after strongly inhomogeneous radiation exposure or after total body exposure with stem cell transplantation can well be considered as a repetition of the embryogenesis of bone marrow hemopoiesis with the key element of stem cells migrating via the blood to stromal sites of the marrow prepared to accept stem cells to home and start their replication and differentiation if the micro-environmental quality permits. In summary, the radiation biology of bone marrow hemopoiesis requires a thorough understanding of the physiology and pathophysiology of structure, function and regulation not only of the process of cellular renewal but also of the intricate infrastructure.