Tegaserod (1-{[(5-methoxy-1H-indol-3-yl)methyliden]amino}-3-pentylguanidine) is a potent agonist at human recombinant 5-HT 4 serotonin receptors. Consequently, tegaserod is utilized in the treatment of bowel diseases. The objective of this study was to test the hypothesis that tegaserod stimulates human cardiac atrial 5-HT 4 -receptors via cyclic adenosine monophosphate (cAMP)-dependent pathways. Tegaserod exerted positive inotropic effects (PIEs) and positive chronotropic effects (PCEs) in isolated left and right atrial preparations, respectively, from mice with cardiac-specific overexpression of the human 5-HT 4 serotonin receptor (5-HT 4 -TG) in a concentration-and time-dependent manner. However, no effect was observed in the hearts of littermates of wild-type mice (WT). Western blot analysis revealed that the expression of 5-HT 4 receptors was significantly higher in 5-HT 4 -TG mice compared to WT mice. The specificity of the signal for the 5-HT 4 receptor was confirmed by the absence of the signal in the hearts of 5-HT 4 receptor knockout mice. Furthermore, tegaserod increased the force of contraction (at concentrations as low as 10 nM), reduced the time of tension relaxation, and increased the rate of tension development in isolated electrically stimulated (at a rate of 60 beats per minute) human right atrial preparations (HAPs, obtained during open-heart surgery) when administered alone. The potency and efficacy of tegaserod to raise the force of contraction were enhanced in the presence of cilostamide, a phosphodiesterase III inhibitor. The positive inotropic effect of tegaserod in HAPs was found to be attenuated by the 5-HT 4 serotonin receptor antagonist GR 125487 (0.1 µM). The efficacy of tegaserod (10 µM) in raising the force of contraction in HAPs was less pronounced than that of serotonin (10 µM) or isoprenaline (1 µM). Tegaserod shifted the concentration-response curve of the force of contraction to serotonin to the right in HAPs, indicating that it is a partial agonist at 5-HT 4 serotonin receptors in this model. We propose that the mechanism of action of tegaserod in HAPs involves cAMP-dependent phosphorylation of cardiac regulatory proteins.