Mercury exists as elemental mercury (liquid mercury or mercury vapor), inorganic mercury salts (mercurous or mercuric), and organic mercury compounds (aryl- or alkylmercury). Due to the different chemical and physiological properties of all forms of mercury the effects of mercury on humans shows quite different clinical pictures. In this paper, mercury vapor and inorganic mercury poisoning are mainly reviewed. Liquid mercury, since it is poorly absorbed from the gastrointestinal tract, is nontoxic. Mercury vapor (Hg0) is absorbed rapidly from the alveolar membrane by inhalation and is accumulated mainly in the brain and kidney. In acute exposure to high concentrations of mercury vapor, the symptoms of respiratory, such as chest pain, cough, hemoptysis, and interstitial pneumonitis occur shortly after inhalation. In chronic exposure to relatively low concentrations of mercury vapor, neurological changes are prominent. The signs and symptoms of mercury vapor poisoning is characterized by gingivitis, intentional tremor and erethism, and in addition, weakness, fatigue, loss of weight, and disturbance of gastrointestinal functions appear unspecifically. Effects of inorganic mercury toxicity manifest in the gastrointestinal tract and kidney after ingestion. Early signs and symptoms appear as pharyngitis, dysphagia, abdominal pain, nausea and vomiting, and bloody diarrhea. Afterward renal failure due to necrosis of the proximal tubular epithelium occurs and develops into anuria and uremia. The occurrences of mercury vapor poisoning due to occupational or accidental exposures and inorganic mercury poisoning due to accidental or suicidal ingestion are reducing today in Japan.