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Double-labelled [methyl-14C,5-3H]CDPcholine has been synthesized and subjected to a pharmacokinetic analysis in several biological systems. In transport experiments with intact human erythrocytes no incorporation of radioactivity is observable. On the other hand the results obtained with perfused rat liver suggest a rapid cleavage of the pyrophosphate bridge of the molecule, followed by a rapid uptake of the hydrolytic products. The plasma half-lives of intravenously injected CDPcholine and of its metabolites have been evaluated within 60 sec range. Renal and fecal excretion of the injected radioactivity is negligible: only 2.5% of administered 14C- and 6.5% of the 3H- is excreted up to 48 hr after administration. Liver and kidney are the major CDPcholine metabolizing organs, characterized by a fast and extensive uptake of choline metabolites, followed by a slow release; conversely the rate of uptake of both 3H and 14C-labelled moieties by rat brain is significantly slower, reaching a steady-state level after 10 hr. The characterization of the labelled compounds detectable in the investigated organs provides some insights on the metabolism of the drug: the 3H-cytidine moiety in all the examined organs appears to be incorporated into the nucleic acid fraction via the cytidine nucleotide pool; the [14C]choline moiety of the molecule is in part converted, at the mitochondrial level, into betaine which accounts for about 60% of the total 14C-radioactivity associated with liver and kidney 30 min after administration; [14C]betaine in turn acts as methyl donor to homocysteine yielding [14C]methionine subsequently incorporated into proteins; the time dependent increase in labelled phospholipids is indicative of a recycling of the choline methyl-groups in this lipid fraction via CDPcholine and/or S-adenosylmethionine; the rather extensive amount of labelled methionine detectable in brain probably arises from its uptake from the blood stream, since the enzyme catalyzing the conversion of betaine into methionine is lacking in brain.