In human heart serotonin (5-HT) exerts pleiotropic effects, which are thought to be mediated via 5-HT4 receptors. Here, transgenic mice that overexpress the human 5-HT4a receptor were generated. Contractile studies were performed in isolated electrically driven left atrial preparations and spontaneously beating right atrial preparations of TG and wild type (WT). 5-HT increased force of contraction and phospholamban phosphorylation only in atrial preparations from TG but not from WT. The contractile effects of 5-HT in left atrial preparations could be blocked by 5-HT4 receptor-specific anatagonists. Only in right atrial preparations from TG but not WT, 5-HT exerted a positive chronotropic effect that could be attenuated by 5-HT4 receptor-specific antagonists. Moreover in left atrial preparations of TG a higher incidence of arrhythmias occurred compared to WT. Finally, intravenous infusion of 5-HT increased in vivo left ventricular pressure and its first derivative in TG but not in WT. The data suggest functional expression of human 5-HT4 receptors in the heart of TG mice.