The aim of the present study was to investigate systematically the effects of noxious stimulation applied to the oral-facial region on visceromotor responses to colorectal distention. In the present study an electromyogram (EMG) of the external abdominal oblique muscle evoked by colorectal distention was measured as a visceromotor reflex response, and inhibitory effects of noxious stimulation were estimated by the decrease of EMG activity. Under halothane anesthesia (1 % in air), colorectal distentions (60 or 80 mmHg) were produced by inflating a balloon inside the descending colon and rectum. Innocuous brush and press stimuli applied to the facial skin did not induce any change in EMG responses evoked by colorectal distention. Noxious pinch stimuli produced a profound reduction of EMG responses during application of pinch. This inhibitory effect remained even after the cessation of pinch stimulation. Pinch stimulation was effective both ipsilaterally and contralaterally indicating a bilateral effect. When the inhibitory effects of ipsilateral pinch stimulation at various parts of the oral-facial region were examined, no significant difference in the inhibitory effect was observed among the oral mucosa, the tip of the tongue and the root of the tongue. A comparison of the inhibitory effects of ipsilateral pinch stimulation among the areas innervated by three branches of the trigeminal nerve showed no significant difference in the inhibitory effect among areas. Electrical stimulation of the tooth pulp (1.6 mA, 50 Hz, 0.1 ms pulses) also reduced the EMG responses to colorectal distention. Following the application of naloxone (2 mg/kg, i.p.), an opioid receptor antagonist, the decreased EMG responses during pinch stimulation were reversed. These results suggest that noxious stimulation to the oral-facial region can suppress visceromotor reflex responses to colorectal distention and that this suppression is mediated by the opioid system. A possible mechanism for trigeminal suppression of visceromotor function is discussed.