The effects on morphologically and electrophysiologically characterized myenteric neurons of activation of intestinal reflex pathways were examined in vitro. Opened segments of guinea pig small intestine were pinned serosa down in an organ bath that had two balloons set into its base. A 5-10-mm-wide strip of myenteric plexus between the balloons was exposed from the mucosal side, and neurons were impaled with microelectrodes. Reflex pathways were stimulated by inflation of the balloons to distend the intestinal wall, and by deforming the exposed mucosal villi with a brush. Impaled neurons were classified electrophysiologically as AH-neurons or S-neurons (Hirst et al., 1974) and were injected with biocytin to determine their shapes and projections. None of the 58 AH-neurons responded to distension. In contrast, 63 of 131 S-neurons responded to distension with a burst of fast EPSPs; about one-third of the responding S-neurons received input from ascending reflex pathways, one-third received input from descending reflex pathways, and one-third received input from both ascending and descending pathways. Most neurons in this last group supplied extensive varicose branches to the tertiary plexus and were probably longitudinal muscle motor neurons. Neurons receiving input from only one pathway usually projected in the direction of that pathway; many of these were circular muscle motor neurons. Almost all neurons responding to distension were also excited by deforming the villi. Responses evoked by distension or deforming the mucosa declined when stimuli were repeated at intervals less than 10 sec. This was seen in ascending and descending pathways but was more prominent in the former. Deforming the mucosa evoked a normal response even when the response to repeated distensions had disappeared. It is concluded that distension and deforming the mucosa excite separate populations of sensory neurons to activate reflex pathways that converge onto common motor neurons and probably onto common interneurons.