We investigated the expression of inositol 1,4,5-trisphosphate (InsP3) 3-kinase mRNA after a single electroconvulsive shock (ECS) with in situ hybridization histochemistry in rat brain. At 6 h after ECS, the expression was markedly decreased in the dentate gyrus, and the decrease was maintained until 9 h with a slight recovery. The InsP3 3-kinase mRNA content returned to basal levels after 12 h. We could not detect any apparent changes in the expression of InsP3 3-kinase mRNA in the CA1-CA3 areas of hippocampus, the striatum, and the cerebral cortex at any time point examined. In the temporal pattern, the reduction of the expression in the dentate gyrus was preceded by the induction of c-fos after ECS. These observations suggest that the InsP3 3-kinase might be one of the genes whose expression can be altered by ECS.