1. Paired patch-clamp recordings were performed in cerebellar slices to study IPSCs evoked by interneurones (stellate and basket cells) in Purkinje cells. 2. IPSCs were first examined while keeping the presynaptic cell in the cell-attached mode. In some of these experiments IPSCs had a remarkably broad amplitude distribution. Large variabilities were associated with large mean IPSC amplitudes and were preferentially obtained with presynaptic basket cells. Action currents recorded from the presynaptic interneurone in the cell-attached mode were reproducible indicating that the variability occurs downstream of action potential generation. 3. The variability of IPSC amplitudes was observed independently of the instantaneous firing rate of the presynaptic cell, and could therefore not be ascribed to synaptic fatigue. 4. Highly variable IPSC amplitude were also obtained if the presynaptic interneurone was placed in whole-cell recording with a potassium-based intracellular solution. 5. If the recording pipette were filled with Cs+ ions a large increase in the mean of the IPSCs was observed within seconds after establishing the whole-cell recording. In Cs(+)-dialysed cells it was possible to modulate the mean IPSC amplitudes by altering the characteristics of the presynaptic stimulation. IPSC amplitudes obtained in response to presynaptic voltage pulses to 0 mV were large and had little scatter. IPSCs obtained in response to pulses near +60 mV, close to the reversal potential of presynaptic Ca2+ currents, had a much lower mean amplitude and also had little scatter. Thus presynaptic application of Cs+ ions both increases the mean amplitude and decreases the variability of the postsynaptic response. 6. To test whether interneurones could be coupled by electrical synapses, paired recordings were performed from neighbouring interneurones. No correlation was found between the firing patterns of such paired recordings, indicating that electrical coupling among presynaptic neurones is not responsible for large IPSC fluctuations as recorded in Purkinje cells. 7. Finally, IPSC fluctuations were investigated in paired recordings from two Purkinje cells. IPSCs corresponding to the activity of common interneurones were identified on the basis of temporal correlation. By plotting the amplitudes of such common IPSCs in one cell against those obtained simultaneously in the other cell, the pattern of IPSCs due to a single presynaptic neurone could be identified. These results show that fluctuations of IPSCs due to the same interneurone in one postsynaptic Purkinje cell are independent of those occurring in another Purkinje cell. 8. The results indicate that the major source of fluctuations is localized within the axonal arborization of presynaptic interneurones. The results with presynaptic Cs+ require that the fluctuations involve the concerted release of several presynaptic vesicles. Two possible mechanisms for such multiquantal events are discussed: fluctuations in presynaptic depolarization, and fluctuations in a regenerative Ca2+ amplification mechanism.