The pattern of transverse branching in the olivocerebellar projection to the x zone in the vermis and the lateral c1 zone in the paravermis of the cat anterior lobe was studied using a combined electrophysiological and retrograde double-labelling tracer technique. Fluorochrome-tagged latex microspheres were well suited for this purpose. The results show that the region of olive that supplies climbing fibres to the two zones forms a continuous, rostrocaudally directed column about 2.25 mm in length, in a caudo-lateral to rostromedial part of the medial accessory olive (MAO), on average between A-P levels 12.50-10.50. This column may be divided into caudal and rostral halves that project respectively to the x and lateral c1 zones in the apical folia of lobules V/VIa. Partial overlap between these two territories occurs in an intermediate region (A-P levels 12.00-11.00) in middle MAO where olive cells that supply climbing fibres to either x or lateral c1 are intermingled with a smaller population of cells whose axons branch to provide climbing fibres to both zones. Quantitative analysis showed that, when different tracers were injected into each zone in the same animal, double-labeled cells represented only 5-7% of either single-labelled cell population within this area of overlap. It is concluded that, although some transverse branching is present within the olivocerebellar projection to the x and lateral c1 zones in the apical folia of lobule V, such branching is not extensive.