Fish contain methylmercury and the potentially protective element, selenium. Blood and brain concentrations of these elements were determined in female rats after consuming AIN-93-based diets containing 0.06 or 0.6 ppm of selenium (Se) and drinking water containing 0, 0.5, or 5 ppm of mercury as methylmercury (MeHg) for 6 or 18 months. Brain and blood concentrations of mercury and selenium were also evaluated in neonates after gestational exposure. For adult rats in the high-Se, high-Hg condition, brain selenium content was 0.35 ppm and 1.8 ppm after 6 and 18 months, respectively, but for every other adult-onset condition, it was 0.1 ppm. Blood selenium varied less than two-fold despite a 10-fold difference in diet. After 6 months, mercury content in the brain showed a greater than 10-fold difference between the mercury groups, and interacted somewhat with dietary selenium. After 18 months, no mercury was detected in the brains of the 0.5 ppm groups, and their blood mercury also fell. For the 5.0 ppm groups, brain mercury increased slightly (low Se diet) or several-fold (high Se diet) over that seen at 6 months, and blood mercury also increased. Neonatal selenium concentrations were more labile than adults, and mercury in neonates was generally higher. All animals exposed to 5 ppm of mercury experienced a molar excess of mercury over selenium. Animals exposed to 0.5 ppm mercury showed a balance between mercury and selenium or a selenium excess, depending on the condition.