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Department of Biology, University of York.

1. 5-Methyluridine perfused through the lumen of isolated loops of rat jejunum gave rise to serosal thymine as the major species transported across the epithelial layer; no serosal 5-methyluridine was detected. 2. At 0.1 mM-luminal 5-methyluridine there was enhanced transmural transport of thymine (P less than 0.001) when compared with either 0.1 mM-thymine or 0.1 mM-thymidine as the luminal substrate for thymine transport. 3. At low luminal concentrations (0.025 mM) thymine was a significantly better substrate (P less than 0.001) for the transport of the free pyrimidine than either of the two nucleosides (thymidine or 5-methyluridine). 4. Luminal deoxyuridine gave rise to uracil as the major species appearing in the serosal secretion. High luminal concentrations of deoxyuridine (0.5 and 1.0 mM) gave rise to low levels of the nucleoside in the serosal fluid. 5. As a consequence of the specificity of the mucosal phosphorolysis the ribonucleosides are favoured over the deoxyribonucleosides as substrates for transmural pyrimidine transport.