The spatial distribution of branches from climbing fibre afferents to the cerebellar cortex has been studied using electrophysiological techniques. Recordings were made from a large number of individual Purkinje cells in which climbing fibre responses were evoked by stimulation of the cerebellar surface. For each cell one or more distinct low threshold points were found from which such responses were evoked via an axon reflex pathway. For each climbing fibre studied in this way, from one to three terminations were located. These were frequently very widely distributed in the rostro-caudal direction, but were generally found to lie within a sector of the cerebellar cortical sheet which was very narrow in the medio-lateral direction. A method for estimating the conduction times in the parent axon and in each of its branches is described and applied to the experimental data relating to one axon. Quantitative measurements were made to define the parameters necessary to activate climbing fibre terminals by stimulation of the cortical surface. Using a cerebellar stimulus whose parameters were chosen on the basis of these measurements, the pattern of climbing fibre axon reflex connexions was determined over most of the accessible cerebellar surface by mapping the climbing fibre evoked potentials. Connexions formed by branching climbing fibres were found throughout the accessible cortex. They appeared always to be orientated at light angles to the long axes of the cerebellar folia, whatever the orientation of the folia with respect to the sagittal plane of the animal.