The purpose of these experiments was to systematically examine the characteristics of the excitability change occurring after the inactivation period evoked by the climbing fiber input to Purkinje cells. Ninety-eight Purkinje cells were isolated extracellularly in unanesthetized decerebrate cats. Simple spikes and complex spikes were discriminated separately. Post-stimulus time histograms were constructed from 100 consecutive trials triggered by the occurrence of spontaneous complex spikes. Seventeen Purkinje cells exhibited a reduction of simple spike discharge rate following the inactivation period. However, 14 cells showed no change in simple spike activity, and in 67 cells the discharge rate increased. These changes in excitability following a spontaneous complex spike were independent of the tonic simple spike activity of the Purkinje cell. Single traces of spike train data from Purkinje cells showed that the change in discharge rate was variable, some complex spikes being followed by an increase and others by a decrease in activity. The basis for these observations and the differences between these data and those from studies in which the climbing fiber input was evoked by electrical olivary stimulation are discussed.