Degradation effects of bis (methylmercuric) selenide (BMS) by the protein-SH and reduced glutathione in the liver and brain homogenates of non-treated rats were investigated in the reaction system consisting of the liver homogenate of selenite-treated rat and methylmercury. The protein-SH in the liver and brain homogenates was found to be able to decompose BMS. However, it was revealed that the reduced glutathione in the liver and brain homogenates was independent of the degradation of BMS. The degradation ability of the protein-SH of the liver homogenate was approximately the same as that of the protein-SH of the brain homogenate. On the other hand, when methylmercury was added to the liver homogenate mixture from selenite-treated and non-treated rats or when BMS formed in the mixture of the liver homogenate from selenite-treated rat and methylmercury was decomposed by the addition of the liver homogenate of non-treated rat, an approximately equal amount of BMS was found. The same result was observed when the brain homogenate was used instead of the liver homogenate. These results indicate that the formation and degradation of BMS repeatedly occur in the mixture of the liver homogenate of selenite-treated rat and the liver or brain homogenate of non-treated rat added with methylmercury.