'Noxious' stimuli are pain-producing stimuli and have been traditionally defined as tissue damaging. This definition is not valid for visceral stimuli. Hence, behavioral definitions are needed. Awake, unanesthetized rats learned to passively avoid colorectal distension. The rate of acquisition increased as a function of the distending pressure. Two manipulations, intrathecal morphine and neonatal treatment with capsaicin, attenuated acquisition of avoidance behavior whereas the production of colorectal inflammation using turpentine enhanced acquisition of this behavior. These findings are consistent with the hypothesis that aversion to the stimulus is due to the 'noxious' nature of the stimulus. © 1991.