Data on mercury and selenium interaction in the mammalian body are reviewed. Experimental data from studies on rats show that selenium interacts with mercury metabolism and toxicity after exposure to mercuric mercury. Autopsy data from workers exposed to mercury vapor indicate an association between mercury and selenium retention in the central nervous system, suggesting the formation of a mercury-selenium complex. In animal experiments, mercuric mercury interferes with selenium metabolism and toxicity. Available data do not, at present, permit deduction as to whether additional selenium intake in man, exposed to mercury vapor or mercuric mercury, will have any effect, beneficial or adverse.