Objective: The pathological effects of internal exposure to manganese dioxide-56 (56MnO2) radioisotopeparticles have been previously examined in rats. Here we further examine the effects of 56MnO2, focusingon changes in blood parameters.Materials and Methods: Ten-week-old male Wistar rats were exposed to 3 doses of neutron-activated56MnO2 powder, nonradioactive MnO2 powder, or external 60Co γ-rays (1 Gy, whole body). On days 3 and61 postexposure, the animals were necropsied to measure organ weights and clinical blood parameters,including red blood cell and white blood cell counts; concentrations of calcium, phosphorus, potassium, andsodium; and levels of alanine aminotransferase (ALT), aspartate aminotransferase, amylase, creatinine, urea,total protein, albumin, triglycerides, high density lipoprotein, total cholesterol, and glucose.Results: In the 56MnO2-exposed animals, accumulated doses were found to be highest in the gastrointestinaltract, followed by the skin and lungs, with whole-body doses ranging from 41 to 100 mGy. There were no56MnO2 exposure-related changes in body weights or relative organ weights. The ALT level decreased on day3 and then significantly increased on day 61 in the 56MnO2-exposed groups. There were no exposure-relatedchanges in any other blood parameters.Conclusion: Although the internal doses were less than 100 mGy, internal exposure of 56MnO2 powdershowed significant biological impacts.