Caco-2 cells are a widely used model in drug development to study intestinal drug transport and metabolism. Recently, serotonin (5-hydroxytryptamine, 5-HT) has been characterized as a highly selective substrate of human UDP-glucuronosyltransferase UGT1A6 [Krishnaswamy S, Duan SX, von Moltke LL, Greenblatt DJ, Court MH. Validation of serotonin (5-hydroxytryptamine) as an in vitro substrate probe for human UDP-glucuronosyltransferase (UGT) 1A6. Drug Metab Disp 2003; 31:133-9], an isoform which conjugates planar phenols and is inducible by Ah receptor agonists and by oxidative/electrophile stress. To gain more insight into intestinal 5-HT disposition, uptake and metabolism of this neurotransmitter was studied in Caco-2 cell monolayers. It was found that 5-HT was taken up from the basolateral and to a lesser extent from the apical surface. It was mainly excreted basolaterally as 5-HT glucuronide. 5-HT UGT activity and UGT1A6 mRNA were induced by Ah receptor agonists and by oxidative stress generated by tert-butylhydroquinone and by isomeric thymoquinone, a potential antitumor agent and constituent of Nigella sativa seeds, commonly used as a condiment in the Middle East. While UGT1A6 induction was clearly detectable in NAD(P)H:quinone oxidoreductase 1 (NQO1)-deficient Caco-2 cells, it was not induced in NQO1-efficient HT-29 colon adenocarcinoma cells. The results suggest that--in addition to its detoxification function--intestinal UGT1A6 contributes to intestinal homeostasis of 5-HT from dietary sources and from release by enterochromaffin cells.