Several neuroanatomical and neurophysiological experiments suggest that the hepatic portal vein is not only richly innervated with sympathetic efferents, but also it is an important source of afferent information. By combining retrograde tracing (with True Blue as a marker) and immunological techniques, the cell bodies for the substance P-containing nerves that surround the portal vein and the hepatic artery of the rat have been localized to the spinal sensory ganglia (T8-T13). Since dorsal root rhizotomy abolished all substance P immunoreactive material from nerve fibres that surround these blood vessels, and since no double-labelled cells were detected in the nodose ganglia, an exclusive spinal origin for the substance P-containing sensory nerves is suggested.