To evaluate long-term effects of low doses of selenium and tellurium on the growth and survival of small mammals, rats numbering 102 or more were given 2 µg/ml selenium in drinking water, either as Na2SeO2 or Na2SeO4, and 2 µg/ml tellurium as Na2TeO3. Selenate and tellurite were well-tolerated. Selenite was not, males dying rapidly at early ages and females less so. Growth was depressed. Mice numbering 108 or more were given selenite or tellurite, 2 µg/ml of the element in drinking water. Both elements were tolerated. Growth was enhanced in males given selenite, and somewhat depressed in females. At 15 months of age male mice given tellurite showed some increased mortality and body weights were less than those of controls. Species of animal, sex and valence state of selenium were of importance in evaluating effects of this trace element.