Simultaneous whole-cell recordings were made from the soma and dendrites of cerebellar Purkinje cells in rat brain slices. Sodium action potentials, evoked by either depolarizing current pulses or synaptic stimulation of parallel or climbing fibers, always occurred first at the soma and decreased in amplitude with increasing distance into the dendrites. Simultaneous somatic and axonal recordings showed that these action potentials were initiated in the axon. Outside-out patches excised from the soma and dendrites revealed that the sodium channel current density decreased with distance from the soma. Consistent with this finding, comparable attenuation was observed for evoked action potentials and simulated action potential waveforms when sodium channels were blocked. These results show that sodium action potentials in cerebellar Purkinje cells are initiated in the axon and then spread passively into the dendritic tree.