Minnows were kept in aquaria containing filtered water dosed with measured amounts of mercury as mercuric chloride and selenium as sodium selenate. Fishes exposed to selenium in combination with mercury showed significantly higher survival rates than those kept in tanks containing mercury alone. A 2 to 1 selenium to mercury molar ratio proved to be most effective in reducing mercury toxicity. The presence of selenium tended to increase the uptake of mercury from the water. There was no observed difference in the rate of mercury elimination in the presence or absence of selenium. These results suggest that the observed protective effect of selenium against the toxicity of mercury does not involve reduction of mercury uptake or enhancement of mercury elimination. The presence of mercury did not significantly affect selenium uptake. However, selenium elimination was reduced in the presence of mercury, suggesting that a mercury-selenium complex is formed.