Electrical stimulation has been used to activate climbing fibres (CFs) projecting to the cat cerebellar cortex, and the effects of low frequency (up to 10 Hz) trains of CF inputs on the discharges of cerebellar cortical neurons has been examined. Repetitive activation of the CF innervating a Purkinje (P) cell could reduce or completely suppress the ongoing simple spike discharges (SS) of the P cell. Suppression of SS was very readily produced by trains of short bursts of impulses in the CF. SS could remain suppressed for up to 4 min after such a train of CF impulses. Recording obtained from Golgi, basket and granule cells indicates that these cortical neurones did not mediate the suppression. It is proposed that the SS are suppressed as the result of a direct action of CF impulses on the P cell's dendritic tree which blocks the ongoing transmission of excitation from parallel fibres.