INOSITOL 1,4,5-trisphosphate (Ins(l,4,5)P₃) serves as an intracellular second messenger for several neurotransmitters, hormones and growth factors by initiating calcium release from intracellular stores^1,2. A cerebellar Ins(l,4,5)P₃ receptor has been characterized biochemically^3,4 and shown by immunocytochemistry to be present in intracellular membranes in Purkinje cells⁵. We show that a previously described Purkinje-cell messenger RNA⁶ encodes a protein of relative molecular mass 260,000 (260 K) with the same properties as the cerebellar Ins(l,4,5)P₃ receptor. Its sequence is partially homologous to the skeletal muscle ryanodine receptor⁷. By immunocytochemistry and electron microscopy the protein is shown to be present in all parts of the endoplasmic reticulum, including those that extend into axon terminals and dendritic spines. Our results indicate that gated calcium release from intracellular stores in muscle and Purkinje cells uses similar calcium-channel proteins localized in analogous intracellular compartments. This implies that the intracellular calcium stores in the endoplasmic reticulum of neurons extend into presynaptic terminals and dendritic spines where they may play a direct role in regulating the efficacy of neurotransmission.