We examined specific receptor/transmitter combinations used at functionally identified synapses in ascending and descending reflex pathways of guinea pig distal colon. Excitatory or inhibitory junction potentials (EJPs, IJPs) were recorded intracellularly from nicardipine-paralyzed circular smooth muscle in either the oral or anal recording chamber of a three-chambered organ bath, respectively. Blockade of synaptic transmission in the central chamber with a 0.25 mM Ca⁺⁺/12 mM Mg⁺⁺ solution abolished EJPs evoked by distension applied either in the central or the far (anal) chamber. IJPs evoked by distension in the central or the far (oral) chamber were depressed to about 50% of control. Hexamethonium (nicotinic receptor antagonist, 200 µM) in the central chamber reduced IJPs evoked by far or central distension to 50%, while EJPs evoked by far distension were abolished and EJPs evoked by central distension were reduced to 70% of control. Hexamethonium in the recording chambers reduced both IJPs and EJPs evoked by central distension to about 50%. EJPs in the ascending pathway were unaffected by blockade of muscarinic receptors in the central chamber or blockade of NK₃ tachykinin receptors in this or the recording chamber. In the descending pathway, blockade of P2 receptors in the same chambers had only a minor effect on distension-evoked IJPs. Thus, some intrinsic sensory neurons of guinea pig colon have long descending projections (> 30 mm), but ascending projections of less than 15 mm. In contrast to the ileum, transmission between ascending or descending interneurons and from sensory neurons to descending interneurons is predominantly via nicotinic receptors; but transmission to inhibitory or excitatory motor neurons and from sensory neurons to ascending interneurons involves nicotinic and other, unidentified, receptors.