1. The hypothalamic paraventricular nucleus (PVN) is an important integrative site within the brain composed of magnocellular and parvocellular neurons. It is known to influence sympathetic nerve activity. 2. The parvocellular PVN contains neurons that project to the intermediolateral cell column of the thoraco-lumbar spinal cord (IML). This defines the PVN as an autonomic 'premotor nucleus', one of only five present within the brain. 3. Another projection arising from the PVN is a prominent innervation of the pressor region of the rostral ventrolateral medulla (RVLM), also a premotor nucleus. The distribution of the PVN neurons projecting to the RVLM is similar to that of the PVN neurons that project to the IML. 4. It has been found that up to 30% of spinally projecting neurons in the PVN also send collaterals to the RVLM. Thus, there are neurons in the PVN that can: (i) directly influence sympathetic nerve activity (via PVN-IML connections); (ii) indirectly influence sympathetic nerve activity (via PVN-RVLM connections); and (iii) both directly and indirectly influence sympathetic nerve activity (via neurons with collaterals to the IML and RVLM). 5. In the rat, results of studies using the protein Fos to identify activated neurons in the brain suggest that neurons in the PVN with projections to the IML or RVLM may be activated by decreases in blood volume. 6. In conclusion, the PVN can influence sympathetic nerve activity. Within the PVN are neurons with anatomical connections that enable them to affect sympathetic nerve activity either directly, indirectly or via both mechanisms (via collaterals). Studies that have examined the role of specific subgroups within the PVN suggest that PVN neurons with connections to the IML or to the RVLM may play a role in the reflex changes in sympathetic nerve activity that are involved in blood volume regulation.