The activity of peroxidases and the level of myeloperoxidase in the bone marrow of spontaneously hypertensive rats (SHR) and stroke-prone SHR (SHR-SP) were determined in comparison with normotensive Wistar Kyoto rats (WKY). In the cetyltrimethylammonium bromide extract of bone marrow, the peroxidase activities using guaiacol or Kl as the electron donor of male and female WKY were different from those of SHR and SHR-SP. The peroxidase activity was also separately determined as myeloperoxidase and eosinophil peroxidase by the use of ion-exchange high pressure liquid chromatography. In males, SHR and SHR-SP contained a low activity of eosinophil peroxidase compared with WKY. Bone marrows of female SHR and SHR-SP contained a lower activity of myeloperoxidase, while SHR and SHR-SP possessed a higher activity of eosinophil peroxidase compared with WKY. No change of the level of myeloperoxidase in the bone marrow was observed among male animals. A significant decrease in the level of myeloperoxidase was observed in female SHR and SHR-SP. Therefore, these results indicate that the change in the activity of the peroxidases in the bone marrow is accompanied by the spontaneously hypertensive state.