Summary In the liver of control rats (-Mo) the copper appeared to be concentrated (> 55%) in the supernatant fraction (nuclei and debris < 15%; mitochondria 20%; microsomes 10%), while in the liver from molybdenum-fed rats more copper was found in the nuclei and debris (27%) and mitochondria (27%) fractions and less in the supernatant (60%) in the supernatant (nuclei and debris < 15%; mitochondria 15%; microsomes 10%). The feeding of sulfate had little or no effect on distribution of copper or molybdenum.