1. The cutaneus trunci muscle (CTM) is a thin broad sheet of skeletal muscle that originates bilaterally on the humerus and inserts beneath the dermis of back and flank skin. A nociceptive stimulus applied to the skin elicits a localized reflex contraction in that region of the CTM underlying the site of sensory stimulation. While this "local sign" character of the CTM reflex corresponds to the segmental distribution of the afferent nerves (the dorsal cutaneous nerves, or DCNs) that enter the spinal cord in the lower thoracic and the lumbar levels, the motor output originates entirely from a circumscribed region of the cervical spinal cord. 2. Electrophysiological analysis of EMG activity in the muscle reflexly evoked by direct electrical stimulation of individual DCNs revealed a distinct topographic relationship, in that the shortest latency response of EMG activity in the muscle was consistently located approximately 1.0 cm rostral to the dermatome of the stimulated DCN. 3. Histochemical studies of the CTM show that individual muscle fibers run rostrocaudally, are focally innervated, and in adult rats, are approximately 3.0 cm in length. The major motor nerves exit from the brachial plexus, and functionally they divide the muscle into longitudinal (rostrocaudal) territories, which thus lie orthogonal to the dermatomal pattern of sensory innervation. The localized reflex responses to focal sensory stimuli indicate that the major longitudinal muscle fields contain many "reflex compartments." 4. The compartmentalized nature of the reflex response in the CTM suggests that nociceptive input from any one sensory dermatome has a preferred access to that fraction of the motoneuron pool that supplies the area of muscle underlying that specific region of skin, i.e., there is a sort of "matching" between groups of primary sensory neurons, interneurons, and motoneurons, which relates to the peripheral location of the stimulated nerve endings and of the muscle fibers that are reflexly activated. Although the partitioning of sensory input to motor nuclei has been shown most clearly for monosynaptic Ia connections, the CTM reflex suggests that sensory partitioning may also be demonstrated in a polysynaptic circuit.