A number of elements including zinc, cadmium, mercury and copper will enhance the synthesis of MT, but only zinc and cadmium are deposited with this protein at higher levels than with other proteins in the liver when these are given orally. However, mercury and copper are deposited with MT in the kidney regardless of whether given orally or by injection. As the level of zinc is increased in the diet a corresponding increase of zinc concentration occurs only in the MT fraction of tissue cytosols. Zinc disappears from the MT fraction to the supplementation levels within 3 to 4 days when rats which have been fed a high zinc diet are changed to a zinc deficient diet. The evidence indicates that MT is involved with zinc metabolism. The half-life of MT is longer when cadmium is used to induce its synthesis than when zinc is used. Dietary sulfur deficiency limits the synthesis of MT. Various stresses, in which strenuous exercise, cold environment, and CC14 injection were most effective, and infections were found to enhance MT synthesis, suggesting a role for it in the defense mechanism.