The feeding or 200 to 500 parts per million or molybdenum to rats resulted in a greater percentage of the total liver copper being found in the nuclei and debris and mitochondrial fractions at the expense of the supernatant fraction, while the molybdenum was found to be concentrated in the supernatant fraction. Dietary inorganic sulfate while lowering liver levels of copper and molybdenum appeared to have little or no effect upon their distribution. Dietary molybdenum also caused the relative amount of copper in the supernatant fraction and erythrocytes of blood to decrease, while the increase occurred in the albumin fraction. As with liver, the blood fraction which contained the most molybdenum was the supernatant. Statistical analysis indicated that a quantity whIch was relatively well correlated with molybdenum toxicity as measured by weight gain was a combination of liver molybdenum and copper concentrations divided by body weight. It appeared that the only beneficial effect of sulfate was to lower this quantity. Sulfate excretion or the manner in which rats handled a toxic amount of phenol was found to be unaffected by molybdenum feeding. Earlier reports that dietary molybdenum caused a decrease in cytochrome-c oxidase activity and an increase in erythrocyte fragility could not be substantiated. An injected dose of radioactive molybdenum was found not to be equilibrated with the already present dietary molybdenum in sixteen hours. / Ph. D.