Correlations between catalase activity and the methemoglobin concentration in the blood of acatalasemic, homozygous hypocatalasemic and normal mice were investigated. The formation of methemoglobin by hydrogen peroxide in the red cells with or without the addition of glucose in vitro was also studied. Levels of methemoglobin concentrations in the blood were decreased in the order of female acatalasemic, male acatalasemic, female homozygous hypocatalasemic, male homozygous hypocatalasemic and normal mice. A good negative correlation were observed between the logarithm of catalase activity and methemoglobin concentration in the blood of acatalasemic, homozygous hypocatoalasemic and normal mice. No significant differences in glutathione peroxidase activity and NADH diaphorase activity among acatalasemic and normal mice were observed. The amount of methemoglobin formed by hydrogen peroxide from hemoglobin in acatalasemic mouse red cells with or without the addition of glucose was more than that in normal mouse red cells. The amount of methemoglobin in normal or acatalasemic mouse red cells without the addition of glucose was more than that in red cells with the addition of glucose. From the above results, the methemoglobin concentration appears to be mainly controlled by the blood catalase activity.