Projections of the carotid sinus nerve (CSN) onto medullary neurones were studied with intracellular recording. Three types of postsynaptic potentials (EPSP, EP-IPSP and IPSP) were recorded by stimulation of the ipsilateral CSN. Of the total of 121 neurones, positions of 54 were identified by intracellular dye. The other 67 were positioned by extrapolation. They were distributed over 5 medullary nuclei: (1) nucleus of the solitary tract (NTS); (2) paramedian reticular nucleus (NPR); (3) perihypoglossal nucleus (PXII); (4) lateral tegmental field (FTL); and (5) nucleus ambiguus. Since penetration of microelectrodes and injection of dye into the NTS neurones was difficult, neurones of the other 4 nuclei were examined. The IPSPs were dominant in small NPR neurones, while the EPSPs were dominant in large neurones of the other 3 nuclei. Both the NA and PXII neurones showed forms of a motor type neurone, while the FTL neurones showed various forms. The EPSPs with onset latency as short as 2-4 msec were frequently recorded in different nuclei. This strongly suggests that the CSN projects monosynaptically onto different nuclei in the medulla.