Animals with hereditary abnormalities of hematopoiesis are quite useful in the study of regulatory pathways of megakaryocytopoiesis and platelet formation. Seven such animal models are analyzed here. The Wistar Furth rat has been recently discovered to have reduced platelet number, but large mean platelet volume, and is, therefore, a model of hereditary macrothrombocytopenia. Study of the Wistar Furth rat may help to elucidate the process of platelet formation. Two mouse mutants the S1/S1d and W/Wv, have macrocytic anemia with reduced megakaryocyte number, but normal platelet count. In these mice, the platelet count is maintained by increased platelet production per megakaryocyte. These models demonstrate that factors other than platelet level are monitored in the feedback regulation of megakaryocytopoiesis and platelet production, and further study should lead to a better understanding of the regulation of megakaryocyte size. The Belgrade rat has severe microcytic anemia with decreased megakaryocyte number. Megakaryocyte size is increased, but platelet count is moderately reduced and thus the megakaryocyte-platelet picture resembles that of severe iron deficiency anemia. A more in depth examination of this model should delineate the effects of iron deficiency and hypoxia on megakaryocytopoiesis. The grey collie dog has cyclic hematopoiesis with large asynchronous fluctuations in all blood cell counts at approximately 2-week intervals. Megakaryocytes have not been studied. This model should be a tool to define the relationships between hematopoietic growth factors and differentiation of the various hematopoietic cell lineages. The br/br rabbit has a transient disturbance in fetal megakaryocytopoiesis and brachydactyly due to spontaneous amputation. Further study of this model may provide a better understanding of fetal megakaryocyte development and establish whether an association exists between the abnormal megakaryocytes and the limb amputations. The nude mouse with its severe T-lymphocyte deficiency has been studied to ascertain whether T cells play a regulatory role in normal and acute thrombocytopenia-stimulated megakaryocytopoiesis. The question of whether T cells or their products are responsible for reactive thrombocytosis in chronic inflammation could be examined with this model. These animal mutants have provided and should continue to provide important models for understanding the regulation of megakaryocytopoiesis and platelet production.