The olivocerebellar system is capable of generating synchronous and rhythmic discharges thought to be involved in the precise timing of movement sequences (Llinás 1991). Implicated in olivocerebellar system function is its effects on cerebellar nuclei (CN), which receives input from the inferior olive (IO) and cerebellar cortex and is the proposed locus through which the olivocerebellar system influences motor coordination. IO neurons are electrotonically coupled via gap junctions, which are hypothesized to be the essential neuronal machinery by which rhythmic and synchronous patterns of activity are generated in the olivocerebellar system. However, while in vitro slice studies have demonstrated electrotonic coupling of IO neurons (Llinás and Yarom 1981a), and in vivo studies have demonstrated that rhythmic and synchronous complex spike (CS) activity persists following the blockade of glutamatergic and GABAergic olivary afferents (Llinás and Sasaki 1989; Lang, Sugihara et al. 1996; Lang 2001; Lang 2002), no definitive proof that gap junctions are responsible exists. The experiments of this proposal directly test the importance of gap junctions in the functioning of the olivocerebellar system and its impact on CN firing. By the use of extracellular single-unit multielectrode recordings of Purkinje cell CS in tandem with single unit cerebellar nuclear recordings and intra-olivary drug injections we have determined the following: (1) olivocerebellar synchrony and rhythmicity are generated by electrical coupling through dendro-dendritic gap junctions in the IO; (2) IO leak conductances are capable of modulating both the CS synchrony and rhythmicity patterns; (3) CS synchrony and rhythmicity are temporally related to CN burst transitions and the olivocerebellar activity likely drives this relationship.