The components of the intestinal peristaltic reflex in humans were examined and the neurotransmitters responsible for them identified for the first time i isolated flat sheet segments of intestine. Increasing radial stretch to the caudad end elicited increasing ascending contraction only, whereas increasing radial stretch to the orad end elicited increasing descending relaxation only. Both components were abolished by hexamethonium, implying the participation of cholinergic interneurons in each component. Atropine inhibited ascending contraction only, abolishing the response to low grades of stretch and partially inhibiting the response to high grades of stretch (69% +/- 17%, p less than 0.01). The substance P antagonist [D-Pro2, D-Trp7,9] substance P partially inhibited ascending contraction induced by high grades of stretch only (40% +/- 12%, p less than 0.02). The vasoactive intestinal peptide antagonist [4-Cl-D-Phe6, Leu17]vasoactive intestinal peptide inhibited descending relaxation, abolishing the response to low grades of stretch and partially inhibiting the response to high grades of stretch (40% +/- 4%, p less than 0.001). Release of vasoactive intestinal peptide increased significantly by 91% during descending relaxation only, whereas release of both substance P and substance K increased significantly by 107% during ascending contraction only, supporting the participation of vasoactive intestinal peptide motor neurons in descending relaxation and tachykinin motor neurons as well as cholinergic motor neurons in ascending contraction. The components of the human peristaltic reflex and transmitters regulating them were identical to those found in rat and guinea pig intestine.