It is commonly believed that the cell bodies of mammalian sensory neurons are contained within spinal and cranial sensory ganglia associated with the central nervous system or within the central nervous system itself. However, strong circumstantial evidence implies that some sensory neurons are contained entirely within the gastrointestinal tract. We have investigated this possibility by using intracellular methods to record the responses of myenteric neurons in the guinea-pig small intestine to physiological stimuli applied to the neighbouring mucosa. The results show that the myenteric plexus contains a population of chemosensitive sensory neurons and that these neurons correspond to neurons with AH electrophysiological properties and Dogiel type II morphology. This is the first direct evidence that some sensory neurons are contained entirely within the peripheral nervous system.