Adult male Sprague-Dawley rats received iv injections of 1 mumole of methyl mercury/kg alone or coadministered with 5 mumole of sodium selenite/kg. Tissue concentrations of methyl mercury were determined at 5, 20, and 60 min after treatment. Selenite treatment produced a significant increase in cerebral methyl mercury concentrations and a significant decrease in kidney methyl mercury concentrations at all time points. The concentration of methyl mercury in liver was significantly increased by selenite coadministration at 5 and 20 min but at 60 min after injection the concentration was not significantly different from that found in rats receiving methyl mercury alone. Selenite treatment also significantly lowered blood methyl mercury concentrations at all time points. This decrease was associated with a significant decrease in the concentration of methyl mercury in erythrocytes at 5, 20, and 60 min. Plasma methyl mercury levels at 5 min postinjection were slightly higher in selenite-treated rats but were significantly lower in treated animals at 20 and 60 min. Treatment of rats with selenite did not specifically alter the extent of methyl mercury binding to glutathione in the 108,000g supernatant of cerebrum or in erythrocyte hemolysates. In rats receiving either methyl mercury alone or with selenite, low-molecular-weight methyl mercury complexes could not be detected in plasma 5 min after iv injection. These results suggested that selenite exerted immediate effects on the distribution of methyl mercury in the rat but did not alter the binding of methyl mercury to glutathione in the soluble components of erythrocytes and cerebrum or lead to formation of a persistent low-molecular-weight methyl mercury complex in plasma.