Dendritic spikes enhance the somatic spike output of many neuron types. This is usually manifested by an increase in the number of spikes (transition to bursting, increase in gain) or in an increase in the precision of the spikes for a given input. We have examined the effect of dendritic spikes on spike output in Purkinje cells using simultaneous somatic and dendritic patch-clamp recordings in slices taken from P18–26 rats maintained at 34°C. Stimulation of parallel fibres triggered dendritic calcium spikes which acted to brake somatic firing: as the synaptic stimulation intensity was increased, calcium spikes caused a decrease in somatic output frequency in response to the stimulus, attaining a maximum average firing rate of 214 ± 8 Hz (n = 5) which became independent of the strength of synaptic input. To investigate this effect, synaptic-like current waveforms were injected into the dendrite (>100 µm from the soma) and the somatic spike output was measured. Increasing the injected current resulted in a linear increase in the average somatic sodium spike output (r = 0.98 ± 0.01, slope = 313 ± 35 Hz/nA, n = 5) until the point where dendritic spikes appeared. Increasing the injected current further did not increase the average somatic spike output (r = -0.081 ± 0.2, slope = -17 ± 12 Hz/nA, n = 5, p 0.2) without changing the slope of the sub-dendritic spike regime (r = 0.93 ± 0.04, slope = 308 ± 43 Hz/nA, n = 3, p > 0.9 vs. control). The maximum sustained spike rate reached 290 Hz while the maximum instantaneous spike rate exceeded 650 Hz in penitrem A. Interestingly, both the maximum sustained and instantaneous firing rates under control conditions are comparable to the frequency limit of axonal propagation (1,2), while during BK channel block the somatic spike rates could exceed this limit. Dendritic spikes in Purkinje cells, in contrast to many other neuronal types, therefore clamp the average somatic spike output of Purkinje cells at a rate that ensures that parallel fibre-evoked simple spikes will propagate down the axon. Statistical significance was assessed by Student’s t test, and all data are given as average ± SEM.