When 174 μg labeled cobalt were injected into the jugular vein of a rumen fistula cow about 6% of the dose was present in the blood after several hours with the value falling to 1% or less after 10–15 days. None was found in the rumen contents, about 7% appeared in the feces, and about 65% was found in the urine with large amounts being rapidly eliminated by this path. Very small amounts were found in the milk and saliva. When 174 μg labeled cobalt were introduced directly into the rumen none was detected in the blood. About 82% of the dose was present in the rumen contents after 4 hours with the amount decreasing regularly to less than 1% after 7 days. Over 65% was accounted for in the feces and only extremely small amounts in the urine. No cobalt was detected in the milk or saliva.