Twenty-four h after the subcutaneous administration of 0.5 mumoles selenite labelled with 75Se to rats of 200 g body weight, the retention of selenium at the injection site was significantly increased by the presence of equimolar amounts of methylmercury in the injection solution. The retention of Me203HgCl was not affected by the presence of selenite. The most significant shift caused by interaction was a decrease in the blood content and an increase in the brain content of 203Hg. The brain content of 75Se was also increased to a lesser extent. The shift in the distribution--which was the same whether the two metals were injected at the same site or separately--continuously decreased from 6-48 h. The same interaction pattern was observed when methylmercury and selenite were administered by gastric gavage and differences in distribution increased when the dose was increased from 1.25 mumoles/kg to 2.5 mumoles/kg. The increase in the brain content of mercury caused by selenite was not restricted to simultaneous administration and occurred when selenite was given 2-7 days after methylmercury.