Alvimopan (0.1-1.0 mg/kg; p.o.) partially antagonizes the slowing of small intestinal transit of 113Sn-labelled microspheres in rats. Alvimopan (3 mg/kg; p.o.) has no effect on the visceromotor behavioural responses (VMR) induced by noxious colorectal distension (CRD) in conscious rats.