The neuronal voltage-sensitive calcium channels (VSCCs) are multisubunit complexes consisting of $alpha_1,;alpha_2-delta$ and $eta$ subunits. Heterologous expression and biochemical studies have shown that the activity of VSCCs is regulated by their $eta$ subunits in a $eta$ subunit isoform-specific manner. To elucidate the $eta$ subunit identity of the P/Q-type calcium channel encoded by an $alpha_{1A}$ subunit, which is exclusively expressed in the Purkinje and granule cell of the cerebellum, we have examined the spatial and temporal expression patterns of $eta$ subunits and compared them with those of $alpha_{1A}$ subunit in the developing rat cerebellum. Reverse transcriptase- polymerase chain reaction (RT-PCR) and Northern blot analysis have shown that $eta_4$ subunit mRNA was prominently expressed in the cerebellum and much more abundant than any other distinct $eta$ subunits. RNase protection assay has further demonstrated that the expression of $alpha_{1A}$ and $eta_4$ subunits increased during cerebellar development, while the amount of $eta_2$ and $eta_3$ mRNAs did not significantly change. In addition, a $eta_4$ transcript was present in cultured cerebellar granule cells, but not in astrocyte cells, and the level of $eta_4$ mRNA expression increased gradually in vitro seen as in vivo. Based on the spatial and temporal expression patterns of $eta_4$ subunit, we conclude that $eta_4$ may predominantly associate, but probably not exclusively, with the $alpha_{1A}$ subunit in rat cerebellar granule cells.