Abstract  In order to evaluate biological effects of trace elements, and to ascertain relationships to chronic diseases of man, mice and rats were exposed for their lifetimes to small doses of each of 26 essential and abnormal elements in drinking water, in a laboratory and on a regimen designed to avoid environmental contamination. Growth rates, survival and longevity, microscopic pathology of tissues, concentrations of trace elements in tissues, and in rats, blood pressure, serum cholesterol, glucose and uric acid, aortic plaques and lipids and tumor rates were measured or examined. Surveys of the human environment for 21 elements in foods, water, vegetation, wild animals were also made, by trace element analysis, and human tissue concentrations for 6 elements. As a result of this work, two prevalent human diseases have been reproduced in rats. A model for human arterial hypertension has been developed in rats fed cadmium and a model for human atherosclerosis has been developed in rats deficient in chromium and in those fed refined white sugar. The following elements were carcinogenic: selenium, yttrium, rhodium, palladium and tellurate. These elements were more or less toxic in mice and/or rats: selenite, germanate, cadmium, tin, antimony, tellurium, lead.