Rats were subjected for 2 weeks to separate and combined exposures to mercuric chloride and sodium selenite at doses of 0.5 mg Hg/kg and 0.5 mg Se/kg. The content of mercury, selenium and protein as well as the activities of glutamate dehydrogenase (GLDH) and malate dehydrogenase (MDH) were determined in homogenates, mitochondria and intramitochondrial structures of the exposed animals. It was found that both separate and combined exposures of rats to mercuric chloride and sodium selenite inhibited GLDH activity and did not affect MDH activity in the examined organs. Mercury-selenium interaction brought about a decrease in the content of mercury in the intramitochondrial structures of kidneys and an increased accumulation of both elements in the outer and inner membranes of liver mitochondria. The biochemical mechanism of mercury-selenium interaction is discussed.