48 weaned male Sprague-Dawley rats with an initial average body weight of 41 g were divided into 4 groups of 12 animals (zinc-deficient; zinc-adequate, pair-fed with zinc-deficient group; selenium-deficient; selenium-adequate) for 40 days. All groups were fed a semisynthetic diet with casein being the source of protein. In the selenium-deficient diet, there was a selenium concentration of 0.038 mg/kg. The other diets were supplemented with Na-selenite in order to adjust the selenium concentration to 0.3 mg/kg. In the zinc-deficient diet, there was a zinc concentration of 4.1 mg/kg. The zinc concentrations in the other diets were adjusted to 45 mg/kg by the addition of zinc-sulfate heptahydrate. Zinc-deficient rats were characterized by a markedly reduced alkaline phosphatase activity in their serum, whilst selenium-deficient rats showed a markedly reduced glutathione peroxidase in serum proving their respective zinc-deficient and selenium-deficient states. Zinc deficiency decreased concentrations of triiodothyronine (T3) and free thyroxine (fT4) in serum by approximately 30% when compared with zinc-adequate controls. The concentration of thyroxine (T4) in serum was not affected by zinc deficiency. Selenium-deficient animals had lower concentrations of T3 and T4 than selenium-adequate animals. The concentration of fT4 in serum was not affected by selenium deficiency. The activity of hepatic type I 5'deiodinase was decreased by 67% by zinc deficiency and by 47% by selenium deficiency compared to adequate controls. The study data show that both zinc and selenium deficiency affect the metabolism of thyroid hormones.