Inorganic and organic arsenics were administered to compare and evaluate their toxicities for 4 weeks by oral gavage in male cynomolgus monkeys. Sodium arsenite, an inorganic arsenic compound, was administered at doses of 1 and 5 mg As/kg. Dimethylarsenic acid, a methylated form of arsenic, was administered at dose of 15 mg As/kg. Arsenocholine, an organic arsenic compound, was administered at doses of 1.5 and 15 mg As/kg. Clinical observation, changes of body weights, food consumption, ophthalmic examination, urinalysis, hematology, serum biochemistry, gross observation at necropsy, organ weights and histopathology were performed to evaluate for this purpose. As a results, toxicities of arsenic compounds differed between various types of inorganic (sodium arsenite) and organic arsenic (dimethylarsenic acid and arsenocholine) in cynomolgus monkeys. Four monkeys were dead at dose of 5 mg As/kg in sodium arsenite treated group. Changes in lung (discoloration, thoracic fluid, hemorrhage and pneumonia) and liver (focal necrosis) were observed in sodium arsenite treated group at dose of 5 mg As/kg. In dimethylarsenic acid treated group at dose of 15 mg As/kg, only one monkey was dead without histopathological findings in liver and lung. The clinical signs of gastrointestinal irritation such as vomiting and diarrhea were observed in sodium arsenite treated group at doses of 1 and 5 mg As/kg, and in dimethylarsenic acid treated group at dose of 15 mg As/kg. Body weight loss showed only in sodium arsenite treated group at dose of 1 mg As/kg. In hematological changes, red blood cell, hemoglobin, hematocrit and platelet were decreased in sodium arsenite treated group at dose of 1 mg As/kg and in dimethylarsenic acid treated group at dose of 15 mg As/kg. In conclusion based on our investigations, sodium arsenite of inorganic arsenic shows more severe toxicities than dimethylarsenic acid, its methylated form. On the other hand, arsenocholine, an organic arsenic compound, is thought to be safer than sodium arsenite and dimethylarsenic acid because it showed no toxicity in this study.