The effect of selenium compounds on the transport of methylmercury (MMC) and HgCl₂ was studied by circulating perfusion experiments in an improved in vitro apparatus. When the initial mercury content in the inner solution was 0.5 mM, promotion of mercury transport was observed upon the addition of 0.01 mM selenite. The transport of MMC was greatly increased with addition of increasing amounts of selenite, while in the case of HgCl₂, mercury transport rather decreased with increasing amounts of selenite added. When selenate was added to the inner solution containing MMC, its increasing effect on mercury transport was considerably weaker than that of selenite. Transport of HgCl₂ was slightly increased by the addition of selenate. Accumulation of mercury in the intestine was hardly changed with increasing addition of selenite or selenate in the case of MMC, but increased in the case of HgCl₂. Accumulation of selenium in the intestine was higher with selenite than selenate. The above observations may be closely related to the mechanisms of absorption, distribution, and toxicity of mercury compounds in vivo in the presence of selenium compounds.