The opisthotonos (opt) mutation arose spontaneously in a C57BL/Ks-db^2J colony and is the only known, naturally occurring allele of opt. This mutant mouse was first identified based on its ataxic and convulsive phenotype. Genetic and molecular data presented here demonstrate that the type 1 inositol 1,4,5-trisphosphate receptor (IP₃R1) protein, which serves as an IP₃-gated channel to release calcium from intracellular stores, is altered in the optmutant. A genomic deletion in the IP₃R1 gene removes two exons from the IP₃R1 mRNA but does not interrupt the translational reading frame. The altered protein is predicted to have lost several modulatory sites and is present at markedly reduced levels in opt homozygotes. Nonetheless, a strong calcium release from intracellular stores can be elicited in cerebellar Purkinje neurons treated with the metabotropic glutamate receptor (mGluR) agonist quisqualate (QA). QA activates Group I mGluRs linked to GTP-binding proteins that stimulate phospholipase C and subsequent production of the intracellular messenger IP₃, leading to calcium mobilization via the IP₃R1 protein. The calcium response in opt homozygotes shows less attenuation to repeated QA application than in control littermates. These data suggest that the convulsions and ataxia observed in opt mice may be caused by the physiological dysregulation of a functional IP₃R1 protein.