INHIBITION of the enzyme inositol monophosphatase (IMPase) (E.C. 3.1.3.25) has been linked to the therapeutic action of lithium in the treatment of manic-depression (bipolar) disorder. Because of the link between bipolar and IMPase, we felt it would be of considerable importance to determine the human chromosomal localization of the IMPase gene. Fluorescencein situhybridization analysis using a human cDNA clone, which included the 5′-UTR and the complete coding region, mapped the human IMPase gene to chromosome 8q21.2–21.3. No gene locus for manic-depressive disorder has yet been identified. Further studies on this IMPase gene, and other potential gene variants and mutations, should help to determine if specific subgroups of patients with manic-depressive disorder can be determined on a molecular basis, with regard to the IMPase gene.