Three problems, i. e., the distribution of mercury in the blood after the exposureto mercury vapour, the relationship between the mercury contents in urine, red cells, plasma and hair, and the evaulation of extents of occupational mercury exposure and non-occupational one, were examined on workers exposed to mercury vapour. To es-timate the mercury of inorganic mercury form, Magos and Cernik's method was used for the undigested urine, red cells and plasma. The hair was extracted by a solution of 1 N HCl. The extract was applied to the mercury vapour photometry. By oxidation and application of oxidized materials to the photometry, the total content ofmercury was estimated. A larger amount of inorganic mercury was found in the plasma than in the redcell, but the portion of mercury measurable by oxidation, namely mercury in Hg-C bond or organic mercury, was distributed to the red cell in a much higher content to the plasma. A significant correlation was found between the contents of mercury of inorganic one in the plasma and in the urine, and between the contents of mercury of organic one in the red cell and the hair. Because of the fact that the non-occupational exposure to mercury was mainly due to methylmercury in Japan, the total content of mercury in the red cell or the hair was markedly changed from a hypothetical levelconsidered for the workers exposed solely to mercury vapour.