The present study was undertaken to examine the effects of chemical irritation of the colon on responses to noxious colorectal distension (CRD) in the rat. Pressor and abdominal visceromotor (electromyographic, EMG) responses to CRD were examined in chronically instrumented, unanesthetized rats before and at 6 and 24 h after intracolonic instillation of 5% acetic acid (HAc) or saline. The magnitude of the visceromotor response to phasic CRD (80 mmHg, 20 s) was significantly greater in HAc- than in saline-treated rats at both 6 and 24 h. This was accompanied by a significant increase in the resting EMG activity in both groups, but not between groups. There was, however, no change produced in either the resting mean arterial pressure (MAP) or the magnitude of the pressor response to CRD by HAc treatment. Signs of colonic inflammation (leukocyte infiltration) were examined periodically after HAc treatment, but were apparent only at 24 h. It is concluded that mechanical (distension) and chemical irritation (HAc) of the colon can elicit changes in the basal visceromotor activity and the visceromotor response to CRD. These changes seem to reflect an alteration in the sensitivity of the colon to noxious stimuli.