Sympathetic unit nerve activity was recorded from the cervical sympathetic trunk of anesthetized, vagotomized, and carotid sinus-denervated cats. Single pulses or short trains of pulses with suprathreshold intensity for afferent C fibers were applied to the common peroneal nerve to elicit somatosympathetic A or simultaneous A and C reflexes, respectively. Bilateral lesions of an ascending spinal pathway in the dorsolateral sulcus area (DLS) of the T12 spinal cord eliminated the sympathetic C reflex. The remaining somatosympathetic A reflex was abolished by the subsequent lesion of an ascending spinal pathway in the dorsolateral funiculus (DLF) bilaterally. The duration of the silent period was found to be mainly influenced by an ascending spinal pathway in the DLF. It was concluded that excitation of afferent A fibers activates ascending pathways mainly in the DLF and a smaller contribution in the DLS of the spinal cord. This produces a small reflex excitation (A reflex) followed by a long silent period in the sympathetic nerves resulting in a net decrease of total sympathetic nerve activity. On the other hand, excitation of afferent C fibers activates the ascending spinal pathway in the DLS. This produces a reflex excitation (C reflex) which exceeds the decreased activity due to the silent period resulting in a net increase of total sympathetic nerve activity.