The hemoglobin (Hb) function is to bear oxygen. Hb is under the exposition of high oxygen concentration in the lung capillaries and it is very slowly oxidated to methemoglobin (MetHb) which is not able to bear oxygen. Enzymatic equipment of erythrocyte protects Fe2+ in HEM in front of the oxidation to Fe3+. It is created 3% of MetHb daily. This action is considered to be autooxidation and it is simultaneously reduced by creation of "complex formation" of MetHb and cytochrome b5. The physiological level of MetHb is 0-2% in the peripheral blood. The MetHb level is enhanced by hereditary impairment of the erythrocyte enzyme apparatus or by changed Hb structure. The patients have enhanced MetHb level in peripheral blood in the range of 1-30%. The only symptom which is created is peripheral cyanosis with neither subjective nor objective difficulties. The patients do not require any treatment. They are very sensitive on receipt of oxidative agents. Occurrence of acquired methemoglobinemia is more frequent in clinical praxis. Administration of oxidative agents is the reason of the disease. The most frequent oxidative agents are sulfones, local anesthetics, enhanced contents of nitrates in vegetables from unsuitable manure soil. The acquired methemoglobinemia causes minimal clinical problems depending on the doses of administered drug and abates after its discontinuation quickly. Only occasionally it causes clinically expressive symptoms. The level above 10% of MetHb causes peripheral cyanosis. The level of MetHb higher than 35% causes general symptoms which are results of the tissue hypoxia. The level about 70% of MetHb is associated with coma and may have fatal outcome. When the level of MetHb is not very high it abates after discontinuation of the administration of causative drug. In the case of high level of MetHb the drug of choice is administration of hyperbaric oxygen, methylene blue, ascorbic acid intravenously or riboflavin in high doses. In the case of deficit of glucoso-6-phosphate-dehydrogenase the administration of methylene blue is contraindicated, because of worsen of the methemoglobinemia.