The topographic organization of afferent projections from the deep cerebellar nuclei, medulla oblongata and spinal cord to the paramedian reticular nucleus (PRN) of the cat was studied using the horseradish peroxidase (HRP) method of retrograde labelling. Discrete placements of HRP within each of the dorsal (dPRN) and ventral (vPRN) regions of the PRN showed some segregation of input. The deep cerebellar nuclei project in a predominantly contralateral fashion upon the PRN. A small but significant ipsilateral fastigial afferent component is also present. The fastigial and dentate nuclei contribute the majority of fibers to the dPRN whereas the interposed nucleus provides very little. The vPRN receives a relatively uniform input from all 3 cerebellar nuclei. Both lateral vestibular nuclei contribute the majority of fibers from the vestibular nuclear complex largely from their dorsal division. Additional input arises from bilateral medial and inferior vestibular nuclei. The vPRN receives relatively more fibers from the inferior vestibular nuclei than does the dPRN while inputs from the medial vestibular nuclei are comparably sparse. The PRN receives bilateral projections from the nucleus intercalatus (of Staderini). A significant projection to the contralateral PRN occurs from the ventrolateral subnucleus of the solitary complex and its immediate vicinity. Additional sources of medullary afferent input include the lateral, gigantocellular and magnocellular tegmental fields, the contralateral PRN and the raphe nuclei. Sites of origin of spinal afferents to the dPRN are bilaterally distributed mainly within Rexed's laminae VII and VIII of the cervical cord whereas those to the vPRN are confined largely to the medial portion of the contralateral lamina VI in the C1 segment. A few labelled cells are found in the thoracolumbar cord with those to the vPRN being more caudal. These data provide the neuroanatomical substrate for a better understanding of the functional role of the PRN in mediating cardiovascular responses appropriate to postural changes.