Liver cell proliferation is a complex process that can be affected by a large number of factors such as bile acids, which have been reported to be associated to the pathogenesis of liver cancer. In this work, bile acid–induced modifications in DNA synthesis by regenerating perfused rat liver were investigated. Two-thirds hepatectomy was carried out 24 hr before perfusion of livers with recirculating, erythrocyte-free Krebs-Henseleit solution. The viability of the preparations was maintained under all experimental conditions, as indicated by bile flow, oxygen uptake, perfusion pressure, perfusion flow and release of lactate dehydrogenase and potassium into the perfusate. Livers received (min 10 to min 60) bile acid infusion at a rate of 25 nmol/min/gm liver (i.e., maximal secretion rate/2) in regenerating livers as calculated for taurocholate in separate experiments). Trace amounts of [methyl-14C]thymidine were added to the perfusate at min 30. At the end of the experiments (min 60) the livers were washed, removed, weighed and homogenized to determine radioactivity in whole tissue, in DNA and in non-DNA-related fractions. Taurocholate and, to a lesser extent, taurodeoxycholate and dehydrocholate (but not ursodeoxycholate) were found to reduce 14C incorporation into DNA. This was not due to changes in the content of 14C in whole, regenerating liver tissue. Taurocholate, taurodeoxycholate, dehydrocholate and ursodeoxycholate had no effect on thymidine uptake; moreover, the proportion of 14C found in bile was negligible. However, bile acid–induced modification in the fate of intracellular thymidine was observed. In regenerating livers receiving no bile acid, the 14C carried by thymidine metabolites accounted for about 60% of 14C in whole liver tissue. Taurocholate markedly increased this proportion to about 80%. Reverse-phase high-pressure liquid chromatography revealed that most of this 14C (about 80%) was recovered at the elution time, corresponding to thymidine catabolites rather than to DNA precursors. These results suggest that bile acids induce enhancement of thymidine catabolism that reduces its incorporation into DNA; inhibition in the process of DNA synthesis itself, leading to a subsequent increase in the metabolism of DNA precursors; or both. Moreover, from the diversity in this property for bile acid species it might be inferred that changes in the composition and size of the bile acid pool during liver carcinogenesis or regeneration play a role in the modulation of the proliferative process. (HEPATOLOGY 1993;18:1182-1192).