Digestive symptoms depend on multiple interacting gut stimuli, but integration of visceral afferent traffic is poorly understood. Our aim was to elucidate the contribution of simultaneous intestinal stimuli to conscious perception. In 17 healthy subjects we performed stimulus-response tests of intestinal distensions (1-min duration at 5-min intervals in 8-ml increments) either alone or with a background electrical stimulus, and dose-response tests of electrical stimuli (1-min duration at 5-min intervals in 6-mA steps) either alone or with a background intestinal distension. Perception was measured by a 0-6 scale. Background stimulation markedly increased perception of test stimuli, reducing tolerance from 44±3 ml to 32±3 ml and from 67±6 mA to 33±4 mA. However, whereas gut distensions below the perception threshold did not modify perception of the background stimulus (4±1% change), unperceived electrical stimuli exerted an intensity-related sensitising effect and increased perception of the background distension up to discomfortable levels (111±40% increment). In conclusion, activation of different pools of intestinal afferents produces summative effects on perception, and this sensitising effect can be exerted by unperceived stimulation of mechanoinsensitive intestinal afferents.