Psychophysiological experiments were performed in 9 humans using constant-pressure, phasic, graded distention (30 sec, 20-70 mm Hg) of the sigmoid colon as a visceral stimulus. Reliable cardiovascular (pressor), respiratory and visceromotor responses in addition to reports of pressure/pain sensations were evoked by colonic distension in 8 of the 9 subjects. The pressure/pain sensations were referred to the lower abdomen, lower back and perineum and their intensity quantified using a visual analogue scale. Responses to colonic distension were graded and increased with repeated distensions at the same intensity (60 mm Hg). The area of referral as indicated by subject drawings increased with repeated distensions as did the intensity of the subjects' sensory and affective descriptors of the sensation. Five of the subjects differentiated between 'pressure' and 'pain' sensations evoked by colonic distension; the intensity of the 'pain' sensation accelerated during the distending stimulus whereas the 'pressure' sensation was typically stable or adapting during the distending stimulus.