The olivocerebellar and corticonuclear connections of the forelimb area of the paravermal medial C3 zone were studied in the cat using a combined electrophysiological and fluorescent tracer technique. During an initial operation under barbiturate anaesthesia, lobules IV/V of the cerebellar anterior lobe were exposed and small injections of dextran amines tagged with rhodamine or fluorescein were made into areas selected from four different electrophysiologically defined parts of the zone. The inferior olive and the deep cerebellar nuclei were then scrutinized for retrogradely labelled cells and anterogradely labelled axon terminals respectively. The findings demonstrate a detailed topographical organization within the olivocerebellar projection to the medial C3 zone and provide some evidence for a topographical organization of its projection to nucleus interpositus anterior. Both projections are described at a level of resolution not previously attained in neuroanatomical studies and the results strongly support the notion of a micro-compartmentalization of cerebellar olivo-corticonuclear circuits.