Background Aims: Intestinal mast cell infiltration may participate to abdominal pain in irritable bowel syndrome (IBS) patients. However, the underlying mechanisms remain unknown. We assessed the effect of mast cell mediators released from the colonic mucosa of IBS patients on the activation of rat sensory neurons in vitro. Methods: Colonic mast cell infiltration and mediator release were assessed with quantitative immunoflorescence and immunoenzymatic assays. The effect of mucosal mediators was tested on mesenteric sensory nerve firing and Ca2+ mobilization in dorsal root ganglia in rats. Results: Mediators from IBS patients, but not controls, markedly enhanced the firing of mesenteric nerves (14.7 A? 3.2 imp/sec vs 2.8 A? 1.5 imp/sec; P <.05) and stimulated mobilization of Ca2+ in dorsal root ganglia neurons (29 A? 4 vs 11 A? 4 ; P <.05). On average, 64 of dorsal root ganglia responsive to mediators were capsaicin-sensitive, known to mediate nociception. Histamine and tryptase were mainly localized to mucosal mast cells. IBS-dependent nerve firing and Ca2+ mobilization were correlated with the area of the colonic lamina propria occupied by mast cells (r = 0.74; P <.01, and r = 0.78; P <.01, respectively). IBS-dependent excitation of dorsal root ganglia was inhibited by histamine H1 receptor blockade and serine protease inactivation (inhibition of 51.7 ; P <.05 and 74.5 ; P <.05; respectively). Conclusions: Mucosal mast cell mediators from IBS patients excite rat nociceptive visceral sensory nerves. These results provide new insights into the mechanism underlying visceral hypersensitivity in IBS.