The motor and sensory connections of the cervical vagus nerve and of its inferior ganglion (nodose ganglion) have been traced in the medulla oblongata of 32 adult cats with the neuroanatomical methods of horseradish peroxidase (HRP) histochemistry and amino acid autoradiography (ARG). In 14 of these subjects, an aqueous solution of HRP was applied unilaterally to the central end of the severed cervical vagus nerve. In 13 other cases, HRP was injected directly into the nodose ganglion. Three of these 13 subjects had undergone infranodose vagotomy 6 weeks prior to the HRP injection. A mixture of tritiated amino acid was injected into the nodose ganglion in five additional cats. The retrograde transport of HRP yielded reaction product in nerve fibers and perikarya of parasympathetic and somatic motoneurons in the medulla oblongata. Furthermore, a tetramethyl benzidine (TMB) method for visualizing HRP enabled the demonstration of anterograde and transganglionic transport, so that central sensory connections of the nodose ganglion and of the vagus nerve could also be traced. The central distribution of silver grain following injections of tritiated amino acids in the nodose ganglion corresponded closely with the distribution of sensory projections demonstrated with HRP, thus confirming the validity of HRP histochemistry as a method for tracing these projections. The histochemical and autoradiographic experiments showed that the vagus nerve enters the medulla from its lateral aspect in multiple fascicles and that it contains three major components--axons of preganglionic parasympathetic neurones, axons of skeletal motoneurons, and central processes of the sensory neurons in the nodose ganglion. Retrogradely labeled neurons were seen in the dorsal motor nucleus of X(dmnX), the nucleus ambiguus (nA), the nucleus retroambigualis (nRA), the nucleus dorsomedialis (ndm) and the spinal nucleus of the accessory nerve (nspA). The axons arising from motoneurons in the nA did not traverse the medulla directly laterally; rather, all of these axons were initially directed dorsomedially toward the dmnX, where they formed a hairpin loop and then accompanied the axons of dmnX neurons to their points of exit. Afferent fibers in the vagus nerve reached most of the subnuclei of the nTS bilaterally, with the more intense labeling being found on the ipsilateral side. Labeling of sensory vagal projections was also found in the area postrema of both sides and around neurons of the dmnX. These direct sensory projections terminating within the dmnX may provide an anatomical substrate for vagally mediated monosynpatic reflexes. Following deefferentiation by infranodose vagotomy 6 weeks prior to HRP injections into the nodose ganglion, a number of neurons in the dmnX were still intensely labeled with the HRP reaction product. The axons of these HRP-labeled perikarya may constitute the bulbar component of the accessory nerve.