The higher specific radioactivity of ALL in comparison to UA, which are observed after administration of labelled precursors, changed when, oxonic acid and allopurinol, inhibitors of UA and ALL formation were administrated. To interpret these results, it is worth while to recall that nucleotide metabolism can be represented as a complex sequence including de novo synthesis of nucleotides and their degradation to UA and ALL. UA is both partially exported to the blood and partially transformed into ALL, which is totally exported to the blood. Under normal conditions, the radioactivity of UA and ALL is similar, in the case of14 C-glycine, while in the other cases (after 14C-formate and other precursors), the formation of ALL is so fast, that it exceedes export from liver to serum, and the radioactivity of the precursors accumulates in ALL. When we treated with the oxonic acid and allopurinol, formation of ALL was reduced, became slower than its export, leading to a higher labelling into UA.