Complex spike activity was simultaneously recorded from 96 Purkinje cells in the rat cerebellar cortex. Rostrocaudal complex spike synchronicity bands were studied in crus I, IIa and IIb and in vermal lobule 6c. Detailed analysis in crus IIa revealed that complex spike activity was staggered sequentially with a 20--50 cm/sec 'propagation velocity' in the mediolateral direction, and that such activity was bilaterally synchronous. The 'propagation' of complex spike activity was symmetrical between right and left crus IIa. Temporally, the neurons that aligned in the rostrocaudal direction typically generated complex spikes close to simultaneously. The correlation of complex spike firing was high between crus IIa and crus IIb, moderate between crus IIa and vermis 6c, and relatively low between Purkinje cells in crus I and crus IIa. These results indicate that, whilst discrete boarders exist between different isochronicity bands, these bands do communicate with each other in the mediolateral direction via slow 'propagation waves' that loosely bind their activity. The results indicate that the olivocerebellar system is organized, bilaterally, to take advantage of the timing signals generated at the inferior olive nucleus.