Intracellular recordings were obtained from Purkinje cells in rat cerebellar slices maintained in vitro. Adding tetrodotoxin to the superfusion solution produced a typical pattern of repetitive burst firing consisting of a cluster of action potentials followed by a long hyperpolarization. TTX-induced oscillatory activity was not due to modulation of membrane potential although underlying mechanisms for maintenance of oscillatory activity were influenced by membrane voltage. The mechanism of TTX-induced oscillation was not related to the presence or amplitude of I_h] and could still induce the oscillatory activity after blockade of I_h] by cesium. The result from an experiment in which QX-314 was injected intracellularly strongly suggested that TTX-induced oscillatory firing activity was due to blockade of post-synaptic Na⁺ currents intrinsic to PCs.