(1) Chemical destruction of the inferior olive (ION), or midline section interrupting the climbing fibers (CFs) rapidly resulted in marked modifications of Purkinje cell (P. cell) simple spike (SS) firing rate and pattern. (2) After CF deafferentation, P. cells at first about doubled their SS frequency which further increased for the next 10 min. (3) Besides the increase in the firing rate, the spike train became much more regular, which in part seemed to be linked to mass oscillations of the neuronal circuitry, as revealed by strong oscillations of background noise. (4) After ION destruction CF activity could be supplied for by juxtafastigial (JF) stimulation which reduced SS frequency again while the firing became much less regular. These effects were shown to be due to the all-or-nothing activity of the CF and not to the simultaneous stimulation of mossy fibers (MFs) or P. cell axons. Neither were they ascribable to CF collaterals. The differences between this new powerful inhibitory action of the CF system on the P. cell and the well documented pause mechanism is discussed. (5) A quantitative relationship has been established between complex spikes (CSs) and SS firing rates. A steady 2/sec CS frequency was shown to effectively silence the P. cell. (6) When CF stimulation was discontinued, an "off" effect was described. It consisted of an initial rise in SS frequency developing in 9 sec, and a delayed further increase unfolding in about 10 min. (7) When CF stimulation began, an "on" effect was observed, which evolved with an exponential-like kinetic of very variable time-constant seemingly depending on past history.