BACKGROUND Tegaserod is a novel drug for the treatment of constipation-predominant irritable bowel syndrome. Tegaserod is thought to exert its prokinetic effect as a selective partial agonist of serotonin receptor type 4 (5-HT4) receptors located in the enteric nervous system. It is unknown, however, whether tegaserod interacts with the human serotonin reuptake transporter (hSERT) and the uptake transporters for dopamine (hDAT) and norepinephrine (hNET). Therefore, the aim of the present study was to investigate whether tegaserod inhibits SERT-, DAT-, and NET-mediated transport. METHODS Tegaserod inhibition of SERT-mediated [3H]5-HT and NET- and DAT-mediated [3H]dopamine uptake was measured in human embryonic kidney (HEK) 293 cells stably expressing hSERT, hDAT, and hNET in comparison with untransfected control HEK293-FT cells. RESULTS Tegaserod inhibited SERT-, DAT-, and NET-mediated transport with IC50-values of 11.7, 20.7, and 3.2 micromol/l, respectively, while 100 micromol/l estrone-3-sulfate or taurocholic acid, used as negative controls, failed to inhibit hSERT-mediated transport. Using Dixon plot analysis, inhibition kinetics yielded a non-competitive type of inhibition with an apparent inhibition constant (Ki) of 3.1 micromol/l for SERT-mediated 5-HT transport. CONCLUSIONS In the present study we propose an additional mechanism of action for tegaserod as a serotonin uptake inhibitor. By inhibiting SERT and increasing local 5-HT concentrations in the gut wall, tegaserod might exert its prokinetic action via a synergism between 5-HT4 agonism and low-affinity SERT inhibition.