Purkinje cells generate simultaneous complex spikes as a result of olivocerebellar activity. This synchronization (to within 1 ms) is thought to result from electrotonic coupling of inferior olivary neurons. However, the distance from the inferior olive (IO) varies across the cerebellar cortex. Thus signals generated simultaneously at the IO should arrive asynchronously across the cerebellar cortex, unless the length differences are compensated for. Previously, it was shown that the conduction time from the IO to the cerebellar cortex remains nearly constant at approximately 4 ms in the rat, implying the existence of such compensatory mechanisms. Here, we examined the role of myelination in generating a constant olivocerebellar conduction time by investigating the latency of complex spikes evoked by IO stimulation during development in normal rats and myelin-deficient mutants. In normal rats, myelination not only reduced overall olivocerebellar conduction time, but also disproportionately reduced the conduction time to vermal lobules, which had the longest response latencies prior to myelination. The net result was a nearly uniform conduction time. In contrast, in myelin-deficient rats, conduction time differences to different parts of the cerebellum remained during the same developmental period. Thus myelination is the primary factor in generating a uniform olivocerebellar conduction time. To test the importance of a uniform conduction time for generating synchronous complex spike activity, multiple electrode recordings were obtained from normal and myelin-deficient rats. Average synchrony levels were higher in normal rats than mutants. Thus the uniform conduction time achieved through myelination of olivocerebellar fibers appears to be essential for the normal expression of complex spike synchrony.