BACKGROUND: Perturbations of intestinal serotonergic neurotransmission seem to be involved in bowel dysmotility associated with irritable bowel syndrome (IBS) with diarrhea. Oral administration of probiotics is an emerging strategy to improve IBS symptoms, possibly via influencing local serotonin metabolism and neurotransmission. In the present study, we evaluated the effects of the yeast Saccharomyces boulardii CNCM I-745 (S. boulardii) on intestinal motility and serotonergic receptors. METHODS: Isolated rat ileum was contracted in a cumulative concentration way by serotonin (5-HT), various 5-HT agonists or by acetylcholine to determine their effective concentration 50% (EC RESULTS: The serotonin antagonists 5-HT CONCLUSION: S. boulardii CNCM I-745 possesses relaxant properties on the rat ileum involving the inhibition of 5-HT and more specifically 5-HT