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The Intermedius nucleus of the Medulla (InM) is found within the medulla oblongata in a position lateral to the hypoglossal motor nucleus and ventral to the dorsal vagal nucleus. The lateral border of the nucleus is confined by fibres running in the internal·arcuate fibre bundle. The physical properties ofthe InM are extremely similar in rats and mice with the only major difference being an increase in the length of the nucleus which is in proportion to the increase in length of other nuclei between the two species. The InM contains both glutamatergic and GABAergic neurones. Transgenic reporter mice for glutamic acid decarboxylase (GAD) reveal that the calcium binding protein parvalbumin is predominantly found in neurones which express GAD67 and that nNOS is found in those which express GAD65 and GAD67 to a lesser degree. The calcium binding protein calretinin is found to co-localise with GAD67 but the majority ofthe calretinin immunoreactive neurones do not. Neurones in the InM contain many ion channels which dictate their electrophysiological characteristics. A population of parvalbumin immunoreactive neurones in the InM are also immunoreactive for Kv3 ion channels indicative of a fast firing phenotype. Another population of neurones in the InM are immunoreactive for Kv4 ion channels which would be indicative of a slower firing phenotype. These two phenotypes correlate with those observed through' preliminary electrophyisiological characterisations ofInM neurones. All neurones in the InM appear to contain Kv2.1 which may playa role in the integration of synaptic inputs. Afferent terminals immunoreactive for vesicular glutamate transporter 1 and parvalbumin, and therefore thought to be proprioceptive, target the InM. Within the InM proprioceptive afferent terminals make close appositions to neurones which contain GAD65, GAD67, nNOS, CR or PV. Ofal! of the antigens investigated a slight preference for nitrergic neurones is evident. InM neurones which are contacted by proprioceptive afferent terminals can be split into two groups; lightly contacted and heavily contacted with neurones immunoreactive for all of the investigated antigens being found within each group. The role for the proprioceptive afferent input to the InM and the diversity ofthe neurones therein is yet to be elucidated. Projections from the InM to the NTS have previously been reported from both electrophysiological and neur~anatomical tracing experiments suggesting a possible role for the InM in shaping autonomic responses to postural alterations ofthe head and neck.