The spatial and temporal organization of climbing fibre activation of Purkinje cells, the so-called complex spikes, were studied in the rat cerebellar Crus II folium utilizing a multiple microeletrode recording technique. As many as 32 Purkinje cells could be simultaneously recorded by using a custom-built electronic amplifier system and a special data storage device. Analysis of the auto-correlation activity of complex spikes in any given group of Purkinje cells indicated that activation occurs with a particular rhythmicity having a base firing of 10 Hz. Cross-correlation of spontaneous complex spikes demonstrated, in addition to a particular rhythmicity, an extraordinarily high degree of synchronicity within a particular spatial distribution of Purkinje cells. Thus, Purkinje cells organized in rostro-caudal rows tend to fire within 1 ms of each other for distances as far as 800 microm (the width of a folium) from the 'master' neuron. By contrast, Purkinje cells located medial or lateral to the master neuron showed almost no cross-correlation. Administration of harmaline to the animal increased the degree of auto- and cross-correlation but did not change the spatial order of the distribution of the cross-correlation. The results indicate that the olivo-cerebellar system is organized in such a way that climbing fibre afferents may be activated in a close-to-synchronous and rhythmic fashion. The spatial distribution of these afferents over the cortex is such as to activate rostro-caudal bands of Purkinje cells which tend to fire in a close-to-synchronous manner.