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Mouse kidney and liver were found to increase their levels of radioactivity above that of serum from 2 to 60 min after administration of [6-14 C]orotic acid. In spleen, thymus and brain, the radioactivity level reached a maximum soon after the injection and then decreased, as did that in serum. Sixty minutes after the injection, 44% of the administered isotope dose was found in the kidneys, 22 % in the liver and 0.75% in the spleen. The 14 C activity in liver UTP increased rapidly and then remained constant for 60 min. The ratio between the activities in uridine phosphates and UDP-sugars was 3:4 from 10-60 min after injection. In the liver and kidneys, the RNA 14 C activities at 60 min after injection were 15 % of the activity in their acid-soluble fractions. Intraperitoneal administration was found to be preferable to intravenous administration for studies on nucleotides and RNA in mouse liver, due to the delayed incorporation of the [ 14 C]orotic acid activity into the nucleotide pool.

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