Previous studies have suggested that platelet volume may be primarily regulated by the ploidy distribution of mature bone marrow megakaryocytes. However, earlier investigations from this laboratory using C57/BL mice have shown that in response to acute, severe, or moderate thrombocytopenia, platelet volume is regulated independently of megakaryocyte ploidy. Murine strains, including C57/BL, usually have a modal bone marrow megakaryocyte ploidy class of 16N. In contrast, the C3H mouse has a 32N modal bone marrow megakaryocyte ploidy class. We have examined the platelet count, platelet volume distribution, and bone marrow megakaryocyte ploidy distribution of C3H mice during steady-state thrombopoiesis and after depletion of platelets by antiplatelet serum. The platelet count and volume of normal C3H mice were not substantially different from those of C57/BL mice, but megakaryocyte frequency was marginally greater (p less than 0.05), and the ploidy distribution exhibited a a marked reduction in the proportion of 16N cells (p less than 0.001) and increased relative frequencies of 32N (p less than 0.001) and 64N (p less than 0.01) megakaryocytes. In response to acute severe thrombocytopenia, C3H mice demonstrated an increase in platelet volume equivalent to that previously reported for C57/BL mice, without a subsequent shift in the modal megakaryocyte ploidy class. The relative frequencies of 64N and 128N megakaryocytes increased significantly (p less than 0.005) compared to normal C3H mice, without a change in the frequency of 32N megakaryocytes. These studies indicate that during steady-state thrombopoiesis, a greater proportion of higher ploidy megakaryocytes (32N plus 64N) does not necessarily alter peripheral platelet count or platelet volume. Therefore, it appears that neither platelet volume nor count are primarily regulated by bone marrow megakaryocyte ploidy and that the magnitude of upward regulation of megakaryocyte ploidy is limited.