Despite its widespread use in irritable bowel syndrome (IBS), limited clinical data exist on the effects of mebeverine hydrochloride on gastrointestinal motility. Human motor activity in the small bowel is more reproducible than that in the large bowel; therefore the aim of this study was to determine in the small bowel the effects of oral mebeverine in both IBS patients and in healthy controls. Twelve IBS patients (11 females/1 male, 46 +/- 13 years old)-predominant constipation (IBS-C, n = 6) and predominant diarrhoea (IBS-D, n = 6)-and six healthy controls, underwent continuous 48 h ambulant recording of small bowel motor activity. One low energy (400 kcal) and one high energy (800 kcal) standard meal were administered in each consecutive 24-h period. Subjects received, in blinded fashion, placebo tablets in the first 24 h then mebeverine 135 mg q.d.s. in the second 24 h. Mebeverine had no effect on parameters of small bowel motility in controls. In contrast, in both IBS-C (P = 0.01) and IBS-D (P < 0.05) patients, phase 2 motility index was increased during mebeverine administration. Also, after mebeverine the proportion of the migrating motor complex cycle occupied by phase 2 was reduced in IBS-D (P = 0.01), while phase 2 burst frequency was reduced in IBS-C (P < 0.05). For phase 3 motor activity in IBS-C patients, the propagation velocity was decreased (P < 0.01), and the duration increased (P < 0.01). These findings suggest that mebeverine, in the initial dosing period, has a normalizing effect in the small bowel in IBS, enhancing contractile activity in a similar fashion to 'prokinetic' agents, as well as producing alterations in motor activity consistent with an 'antispasmodic' effect.