The interaction of selenium with methylmercury was investigated in brain of animals labeled with 75SeO2-3 and CH3203Hg+. Brains were fractionated into subcellular components and the cytosol was further fractionated by chromatography on Sephadex G-150 and G-200. The main result of these studies was evidence suggesting a shift of 75Se from the cytosol to the mitochondrial fraction in brain when CH3Hg+ was given. Concurrent equmolar (10 micronmoles/kg) selenite injections increased the uptake of Hg but did not alter 203Hg distribution in brain VSports手机版. Changing the dose of CH3Hg+ from 1 to 38 micronmoles/kg had little effect of Hg uptake (% of dose per g). Gel filtrations on Sephadex G-150 and G-200 revealed that 203Hg in cytosol followed a pattern more closely related to protein (A280) than to 75Se, although a considerable portion of both isotopes eluted with proteins in the void vulume. Assays of whole brain homogenates revealed a slight reduction in glutathione peroxidase activity in CH3Hg+-treated rats which was not seen when equimolar selenite was injected with the CH3Hg+ .