The patterns of innervation of the mucosa by axons of individual primary afferent neurones with cell bodies in the myenteric plexus were studied by mapping sites from which electrical stimulation of the mucosa elicited action potentials (APs) in their cell bodies. Segments of guinea-pig ileum were dissected to reveal the myenteric plexus over half of the intestinal circumference, leaving the mucosa intact over the other half. Intracellular recordings were taken from myenteric neurones located within 1 mm of the intact mucosa. Focal electrical stimuli were applied to the mucosa at multiple locations separated by about 1 mm. Neurones that responded had round or oval cell bodies with several long processes (Dogiel type II) and APs that had an inflection on the falling phase (AH-neurones). Responses consisted of single APs or bursts of APs. Maps of the mucosal projections of 30 neurones were generated. The maximum distances from which individual neurones responded were 7 mm circumferential and 2 mm oral or anal to the cell body with a higher proportion of responses from the oral regions. The areas of intact mucosa calculated to be innervated ranged from 1 mm2 up to approximately 15 mm2 (mean 3.9 mm2; median 2.5 mm2). It is estimated that the areas innervated would be two to three times larger under conditions where part of the mucosa is not removed. Some neurones also responded to a chemical or a mechanical stimulus applied to the mucosa within the electrically mapped area. It is concluded that intrinsic primary afferent neurones have overlapping receptive fields with 230-350 neurones innervating the same region of mucosa.