The distribution of SP-like immunoreactivity in the sensory and sympathetic ganglia, as well as the fiber connection between sensory ganglia and sympathetic ganglia (SG) were studied by the immunohistochemical ABC and HRP methods. A large number of SP-like neurons were found in all dorsal root ganglia (DRG), and a large number of SP-like fibers and terminals were found in both thoracic SG and coeliac-cranial mesenteric ganglion complex (CCMG), but no SP positive perikaryons were seen in those ganglia. The various nerves connected with the CCMG have been cut, the results demonstrate that the SP immunoreactive material in the ganglion were only decreased obviously or disappeared by cutting the DRG associated with the CCMG. After injecting HRP into T7-9DRG, a few of labelled neurons were found in thoracic SG of the relative segments and CCMG. The results mentioned above revealed that SP-containing fibers and terminals present in the SG come from collateral branches of primary sensory neurons in the DRG, the postganglionic fibers from principal ganglion neurons in the SG projecting to the DRG, the connection of reciprocal fibers are present between the DRG and the SG. It is suggested that a reflex shortloop outside the central nervous system may be formed directly by primary afferent neuron and sympathetic postganglionic neuron for the purpose of regulating the activity of autonomic nervous system and, moreover the sympathetic nerve may have a feedback regulation function to the primary afferent. The results mentioned above offer the partial morphological testimony for the expoundity of acupuncture mechanism.