To investigate the influence of the brain-gut interactions on the pathophysiology of irritable bowel syndrome (IBS), we compared such patients (n = 10) with healthy control subjects (n = 11) by measuring the pressure of the colon and small intestine simultaneously with analysis of power spectrum of the electroencephalography (EEG) under mental stress and administration of neostigmine. Stress slightly increased the colonic motility index, reduced the percentage of alpha power, and increased the percentage of beta and theta power of the EEG in the patients with IBS more than in the controls (p < 0.05). The patients with IBS had a longer phase II (p < 0.01) and shorter phase I (p < 0.02) of fasting duodenal motor activity than the controls. Neostigmine (10 micrograms/kg) caused a significant difference in the colonic motility index (p < 0.01) and power spectra of EEG (p < 0.05) in the patients with IBS compared to the controls. Significant positive correlation was detected between colonic motility and power spectral change induced by stress (r = 0.46, p < 0.05) or neostigmine (r = 0.51, p < 0.01). These results suggest that patients with IBS have exaggerated responsivity of the gut and the brain to mental stress and cholinergic stimulation. Moreover, there is a possibility that these exaggerated responses are related.