Experiments were undertaken to assess whether the occurrence of a climbing fiber input to a Purkinje cell alters the manner in which the same neuron responds to subsequent mossy fiber input evoked by natural peripheral stimuli. These experiments were based on previous studies showing that the temporal patterning as well as the excitability of a Purkinje cell can change markedly following the occurrence of climbing fiber input to a Purkinje cell. The experiments were performed in decerebrate unanesthetized cats using conventional electrophysiological techniques to record extracellularly the simple and complex spikes from individual Purkinje cells. To test the hypothesis that the climbing fiber input altered the responsiveness of a Purkinje cell to subsequent mossy fiber inputs, poststimulus histograms were constructed of the simple spike and complex spike activity of the Purkinje cells evoked by approximately 1-mm flexion of the forepaw at specific times following the occurrence of a spontaneous climbing fiber input to the same neuron. The computation of the poststimulus time histogram was initiated at the occurrence of spontaneous complex spikes. The same complex spike was used to trigger the peripheral stimulus at specific intervals. Variation in the response to mossy fiber inputs evoked at different intervals following spontaneous climbing fiber inputs was examined. The results demonstrated that preceding the peripheral stimulus by a climbing fiber input at short intervals (20-70 ms) could augment the amplitude of the simple spike response independent of whether the response consisted of an increase or a decrease in discharge rate. For many cells a response to the peripheral stimulus was only observed when the flexion of the forepaw occurred at short intervals following the climbing fiber input. In addition to these short-term alterations in the responsiveness of a Purkinje cell, conditioning the response to the peripheral stimulus at short intervals occasionally produced modification in the simple spike discharge that persisted when the climbing fiber-stimulus interval was returned to 300 ms. These findings demonstrate that the occurrence of a climbing fiber input to a Purkinje cell alters its responsiveness to mossy fiber inputs activated by natural peripheral stimuli. Several implications of these findings are discussed, including a reexamination of the concept of somatotopic distribution in the cerebellar cortex and the importance of the combined activation of mossy and climbing fiber inputs to the cerebellar cortex in determining the distribution of Purkinje cells responding to any given mossy fiber input.