Selenium has been shown to alter mercury metabolism and to decrease mercury toxicity. Parizek and his colleagues (1, 2) demonstrated that injection of selenite markedly diminished the toxicity of simultaneously administered mercuric chloride while paradoxically appearing to cause increased retention of the mercury. Ganther el al.( 3 ) reported that chronic methylmercury toxicity was decreased by feeding 0.5 ppm selenium. Recently Potter and Matrone (4) confirmed the findings of Ganther et al. using a dietary selenium concentration of 5 ppm and showed that this amount of selenium also decreases chronic inorganic mercury toxicity.Little is known of the mechanism by which selenium decreases mercury toxicity.