1. The receptive fields were determined for 118 afferent fibres in the S2, S3 and Ca (caudal) 1 ventral roots of the cat. Of these fibres, ninety‐three were unmyelinated, another eleven were probably unmyelinated, and fourteen were myelinated, according to estimates of their conduction velocities. 2. Confirmation that the recordings were from ventral root filaments came from electron microscopic inspection of ten of the filaments from which recordings of the activity of unmyelinated afferents were made. 3. Receptive fields were demonstrated for twelve unmyelinated afferent fibres in the distal stumps of the S2 and S3 ventral roots which had been sectioned 3 weeks previously, indicating that the cell bodies giving rise to these fibres were not in the spinal cord. 4. The action potentials of some of the unmyelinated ventral root afferent fibres were complex, suggesting branching of the afferents within the ventral root. 5. One third of the unmyelinated ventral root afferents had receptive fields in somatic structures: the skin and deep tissues. 6. Two thirds of the unmyelinated ventral root afferents had receptive fields in the viscera of the pelvis: the bladder, urethra, vagina, and lower bowel. 7. Many of the unmyelinated afferents in the ventral roots, especially those with cutaneous receptive fields, had high thresholds and may participate in nociception. 8. It is concluded that the cat ventral root contains a major sensory component and that the Law of Bell and Magendie is not an accurate description of the organization of the ventral root in this animal.
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