Several studies have suggested that the cerebellum has an important role in timing of sub-second intervals. Previous studies using transcranial magnetic stimulation (TMS) in order to test this hypothesis directly have produced inconsistent results. Here we used 1 Hz repetitive transcranial magnetic stimulation (rTMS) for 10 min over the right or left cerebellar hemisphere to interfere transiently with cerebellar processing in order to assess its effect on the performance of a finger tapping task. Subjects tapped with their right index finger for 1min in time (synchronization phase) with an auditory or visual cue at 0.5, 1, or 2 Hz; they continued for a further 1min at the same rate with no cues (continuation phase). The blocks of trials were performed in a random order. rTMS of the cerebellum ipsilateral to the movement increased the variability of the inter-tap interval but only for movements at 2 Hz that were made while subjects were synchronizing with an auditory cue. There was no effect on the continuation phase of the task, when the cues were no longer present, or on synchronization with a visual cue. Similar results were seen after stimulation over the contralateral dorsal premotor cortex but not after rTMS over supplementary motor area. There was no effect after rTMS over the ipsilateral right cervical nerve roots or over the ipsilateral primary motor cortex. The results support the hypothesis of neural network for event related timing in the sub-second range that involves a cerebellar-premotor network.