Inositol monophosphatase (IMPase) catalyzes the hydrolysis of inositol monophosphate to inorganic phosphate and inositol. For this catalytic process to occur, Mg2+ cations must exist in the active site. According to the inositol depletion hypothesis, IMPase activity is assumed to be higher than normal in patients suffering from bipolar disorder. Treatment with Li+, an inhibitor of IMPase, reduces its activity, but the mechanism by which lithium exerts its therapeutic effects is still at a stage of conjecture. The Escherichia coli SuhB gene product possesses IMPase activity, which is also strongly inhibited by Li+. It has significant sequence similarity to human IMPase and has most of its key active-site residues. Here we show that by using Li-7 magic-angle-spinning solid-state NMR spectroscopy, including {C-13}Li-7 dipolar recoupling experiments, the bound form of lithium in the active site of wild-type E. coli SuhB can be unambiguously detected, and on the basis of our data and other biochemical data, lithium binds to site II, coupled to aspartate residues 84, 87, and 212.