The uptake of a natural nucleoside, thymidine (THY), and one of its deoxynucleoside analogues, zidovudine (AZT), was studied in a newly developed intestinal epithelial model system (CaCO-2). The results of the study indicated that the uptake rate of THY was saturable with a Km of 44.6 microM and maximum rate of 1.73 +/- 0.17 pmol/cm2/s. The uptake of AZT, on the other hand, was not saturable. When various naturally occurring nucleosides were used as inhibitors of THY uptake, it was shown that the uptake was inhibited by various pyrimidine nucleosides (e.g., uridine and cytidine), but not by purine nucleosides (e.g., guanosine). Neither group of nucleosides had any effect on the uptake of AZT. Finally, the uptake of THY was also inhibited by the metabolic inhibitor NaCN and a couple of specific nucleoside transporter inhibitors [i.e., dipyridamole and S-(p-nitrobenzyl-6-thioinosine], whereas none of these compounds had a significant effect on the uptake of AZT. The results showed, for the first time, the existence of a nucleoside carrier in the CaCO-2 cell culture model system. Also the results indicated that the uptake of AZT by the CaCO-2 cell monolayers is mainly via a passive diffusion process.