Most patients with functional bowel disorders complain of daytime symptoms while they remain asymptomatic at night. As symptoms are associated with heightened visceral sensitivity, we hypothesized that circadian fluctuations of the visceral sensory function occur. At four different timepoints (06.00, 12.00, 18.00 and 24.00 h), colorectal distensions (CRD) were performed in fasting conscious male Lewis rats using a balloon catheter and a barostat device. The abdominal wall contractions (behavioural pain response) were assessed during colorectal distension by abdominal wall electromyography (EMG). Plasma levels for endogenous cortisol were determined simultaneously at these timepoints. EMG responses to CRD were significantly (P < 0.05) higher at midnight and in the early morning. Plasma cortisol levels peaked in the evening. In night-active Lewis rats, the behavioural pain response to noxious visceral stimulation is augmented at night and fluctuations of visceral sensitivity are accompanied by circadian changes of plasma concentrations of endogenous cortisol. We conclude that there are marked circadian fluctuations in visceral sensory functions. Thresholds are low during time periods of normal behavioural activity. These findings suggest that fluctuation of the sensory functions may be linked to the circadian variability of symptoms in patients with functional GI disorders.